Topic 6 Organic Chemistry I Flashcards
What is a hydrocarbon?
It’s a compound that only contains hydrogen and carbon
What is empirical formulae?
Is the simplest whole number ratio of elements present in one molecule or formula unit of the compound.
What is molecular formulae?
Represents the actual number of atoms of each element in a molecule
What is a general formula?
A formula that represents a homologous series of compounds using letters and number (e.g. CnH2n+2)
What is structural formulae?
A formula which shows the arrangements of atoms in the molecule of a compound but doesn’t show all the bonds between them. (E.g. CH3CH2COCH3)
What is displayed formulae?
A type of structural isomer that shows all the bonds between every atom in the compound.
Define ‘homologous series’
Groups of molecules that have the same functional group and similar chemical properties.
Define ‘functional group’
A group of atoms responsible for the characteristic reactions of a particular compound.
What are the prefixes for compounds up to C10?
C1 - meth-
C2 - eth-
C3 - prop-
C4 - but-
C5 - pent-
C6 - hex-
C7 - hept
C8 - oct-
C9 - non-
C10 - dec-
Steps of nomenclature (IUPAC rules):
- Identify longest carbon chain
- The chain is numbered to give the lowest possible numbers to the substituents. Double and triple bonds take priority in numbering - receiving the lowest possible numbers. When multiple substituents are present, the numbering should give the substituent that comes first alphabetically the lowest possible number.
- Naming the substituents based on their structures - Alkyl groups (methyl,ethyl,propyl), Halo groups (chloro,bromo,iodo,fluoro), functional groups (hydroxyl,amino,carboxyl,aldehyde)
- Functional groups are given priority when determining the suffix of the compound name. Highest priority to lowest: carboxyl group (-COOH), alcohol (-OH), aldehyde (-CHO), ketone (C=O), amine (-NH2).
- Identify multiple bonds - Double bonds are indicated with -ene suffix, triple bonds are indicated with -yne suffix, for compounds with both -en-yn-
- Position of multiple bonds - is indicated by the number before the base name (e.g. 2-butene, where the double bond is between C atoms 2 and 3)
- If the compound forms a ring, cyclo- is used as a prefix
Name the suffixes of the compound that reflects the functional groups:
Alcohol -
Aldehyde -
Ketone -
Carboxylic acid-
Alcohol -ol
Aldehyde -al
Ketone -one
Carboxylic acid -oic acid
What is an addition reaction?
In an addition reaction, the double bond of an alkene partially breaks when the reactant molecule attacks and adds on across it.
What is an elimination reaction?
In an elimination reaction, a small molecule is removed leaving behind an alkene. This is typically a reaction of halogenoalkanes. They are heated with ethanolic NaOH.
What is a substitution reaction?
Is the replacement of one atom or group of atoms in a molecule for another one
What is hydrolysis?
The breakdown of bonds carried out by water
What is an oxidation reaction?
Is an increase in the oxygen content or decrease in the hydrogen content of an organic molecule
What is a reduction reaction?
Is a decrease in the oxygen content or increase in hydrogen content of an organic molecule
What is a polymerisation reaction?
A process involving monomers combining chemically to produce very large chain like molecules, called polymers
Define ‘structural isomerism’
Compounds that have the same molecular formula but different structures (different manners in which atoms are linked)
Define ‘stereoisomerism’
Stereoisomerism occurs when a compound has the same structural connections, but the spatial orientations are different
Where do E/Z isomerisms occur and why do they occur?
E/Z isomerisms occur in alkenes. They occur due to restricted rotation about C=C. They only occur if the groups bonded to each carbon atom in the C=C bond are different.
When do Z-isomers occur?
The Z-isomers have the groups with priority together, either both above or below the C=C bond
When do E-isomers occur?
They occur when the groups of priority are on opposite sides of the double bond (diagonal from each other)
How to work out if a molecule is a Z or E isomer?
- For single atoms, a higher atomic mass gives a molecule higher priority (for example, bromine has a higher atomic mass than hydrogen so bromine has higher priority)
- For groups of atoms, look at the atom directly bonded to the carbons in the double bond - whichever carbon is bonded to the atom with highest atomic mass has priority. If they are the same, carry down the chain.
- If the groups with higher priority are on the same side it’s a Z isomer
- If the groups with higher priority are on opposite sides, it’s an E isomer
What are Cis-Trans isomerisms?
Cis-Trans isomerism is a special type of E/Z isomerism in which both of the carbon atoms in the C=C group have at least one substituent in common.
Cis isomers have equal groups on the same side
Trans isomers will have equal groups on different sides
What is the general formula for alkanes?
CnH2n+2
What are alkanes and cycloalkanes?
Saturated hydrocarbons
How are alkanes fuels obtained?
Alkane fuels are obtained from the fractional distillation, cracking and reforming of crude oil.
Define reforming
Reforming is described as the processing of straight-chain hydrocarbons into branched-chain alkanes and cyclic hydrocarbons for efficient combustion.
What’s the difference between catalytic cracking and catalytic reforming?
During catalytic cracking, the chemical bonds in large molecules are broken down, whereas, catalytic reforming rearranges the structure of hydrocarbons without a reduction in size.
What is formed during the combustion of alkane fuels?
Pollutants including: carbon monoxide, oxides of nitrogen and sulfur, carbon particulates and unturned hydrocarbons.
What problems are arising from pollutants from combustion of fuels?
- The toxicity of carbon monoxide —> leads to CO poisoning
- Acidity of oxides of nitrogen—> acid rain
- Acidity of oxides of sulfur —> damage to environment
How do catalytic converters solve problems caused by pollutants?
They are used in cars. Precious metals (like platinum) are coated on a honeycomb to provide a large surface. The reactions that take place is the oxidation of unburnt hydrocarbons, oxidation of CO to CO2, reduction of NO to N2.
What are the three main biofuels and how are they made?
- Biodiesel - made by refining renewable fats and oils
- Bioethanol - made by fermentation
- Biogas - made/released when organic waste breaks down
What are the benefits of biofuels?
- Biofuels are considered carbon neutral because when plants grow and absorb carbon dioxide which is equivalent to the amount of carbon dioxide that will be released when they are burnt, however, this isn’t completely correct because carbon dioxide is produced in refining and transport of plants
- Biodiesel and biogas can reduce the amount of waste going to landfill as the waste can be used to produce them
- Biofuel production could provide money for less developed countries as they have the space to grow the crops required
Limitations of biofuels:
- The cost of converting engines and machinery to run on biofuels instead of petrol/diesel
- Many developed countries don’t have the space to be able to produce enough plants to make the biofuels because the land is needed for food production
What is a radical?
A radical is a species with an unpaired electron and is represented in mechanisms by a single dot. It is formed by homolytic fission of a covalent bond and results in the formation of radicals.
What happens in a reaction between an alkane and oxygen in air?
This is a combustion reaction and the products produced is carbon dioxide and water.
What happens in a reaction between an alkane and a halogen?
This forms haloalkanes and so is the process of halogenation. Within this reaction, the free radical reaction mechanism of halogenation will occur. This occurs in 3 steps:
1. Initiation —> involves the formation of two radicals. High temperature/ultraviolet light is a necessary condition. Within the presence of UV, the halogen molecule is broken into two atoms. Each halogen atom takes one electron from the, originally, shared pair of electrons from the covalent bond. This is homolytic fission.
2. Propagation —> involves the reaction between one radical and one molecule. A reactive halogen radical collide with alkane molecule by removing a H atom from the alkane molecule. The alkane radical is formed which is also a reactive radical. It can react with halogen molecules to form the product in the substitution reaction.
3. Termination —> involves reaction between two radicals. Reactive radicals can form a molecule by shared two unpaired electrons in the formation of a covalent bond. Forming stable molecules.
What is the feature of Initiation?
1 molecule —> 2 radicals
What is the feature of propagation?
1 radical + 1 molecule —> 1 radical + 1 molecule
What is the feature of termination?
2 radicals —> 1 molecule
What are the limitations of radical substitution reactions in the synthesis of organic molecules?
- Radical substitution reactions often result in further substitution reactions, leading to formation of multiple products. This makes it difficult to control product distribution, especially when you want a specific product,
- Radical substitution reactions often produce a mixture of products because radicals can attack any H atom on a molecule, leading to different isomers. Making it harder to isolate the desired compound - a particular issue when trying to synthesise a specific compound in high purity,
- Radical substitution requires specific reaction conditions (UV radiation or heat) to initiate. This can cause side reactions or unwanted by-products.
State general formula of alkenes:
CnH2n
What are alkenes and cycloalkenes?
They are unsaturated hydrocarbons
Explain alkenes in terms of sigma and pi bonds:
Alkenes have a double bond which consists of one sigma bond and a pi bond.
Sigma bonds
- formed by head on overlap of orbitals (can be 2 s orbitals, one s and one p orbital, or two p orbitals) between two atoms. In terms of alkenes, sigma bond is formed between two carbon atoms in the double bond.
- Strong bond and allows free rotation around bond axis. However, the free rotation becomes restricted when pi bonds are introduced.
Pi bonds
- are formed by sideways overlapping of p orbitals
- not effective overlapping compared to sigma bonds, so it’s weaker than sigma bonds.
- compounds with pi bonds are usually more reactive than those without pi bonds.
Describe properties of a double bond:
- consists of 1 sigma and 1 pi bond
- cannot be rotated (as pi bond would be broken in rotation)
- two single bond carbons are stronger than one double bonded carbon
- more reactive than single bonds as pi bonds are further away from carbon atoms so they are under less control of carbon atoms and more available for reaction
Describe structure of ethane in terms of it’s sigma and pi bonds:
- three sigma bonds around each carbon atom
- each carbon atom has one electron in p orbitals that has not been used in bond formation
- these p orbitals sideways overlap each other to form pi bonds
- second shared pair of electrons between carbon atoms is formed
Define ‘electrophile’
An electrophile is a chemical species which is attracted to electrons
Explain the addition reaction of an alkene with hydrogen (in the presence of a nickel catalyst)
When alkenes react with hydrogen gas, in the presence of a nickel catalyst, the double bond in the alkene is broken, and the hydrogen atoms are added to the carbon atoms that were part of the double bond. This results in formation of a saturated alkane.
This process is hydrogenation.
Explain the application of additional reactions of alkenes with hydrogen in the manufacturing of margarine
Margarine is made by hydrogenating unsaturated vegetable oils, which contain double bonds between carbon atoms in their fatty acid chains. These oils are unsaturated.
The hydrogenation of these unsaturated oils is performed to convert the double bonds into single bonds, thus making the oil more saturated. Changing the oils physical properties: melting point, allowing it to become solid/semi-solid at room temperature —desired consistency for margarine. This requires a nickel catalyst. By controlling the degree of hydrogenation, margarine can be made as soft or hard as required.
Explain the addition reaction of alkenes with halogens to produce …
A halogenation reaction which in turn produces a dihaloalkane. This process takes place under room conditions readily, without use of any catalyst. Use of organic solvent to dissolve the halogen is required. Bromine is usually used in this reaction to test if a substance is an unsaturated.
Explain the addition reaction of alkenes with hydrogen halides to produce…
This process is called hydrohalogenation as a hydrogen halide (HF, HCl, HBr etc.) molecule is added across a C=C bond, converting the alkene to a haloalkane. However, this test isn’t a good test to test for the presence of an alkene because no colour change can be observed.
Explain the addition reaction of alkenes with steam, in the presence of an acid catalyst to produce…
This process is known as hydration. When an alkene reacts with steam in the presence of a strong acid catalyst (typically phosphoric acid or sulfuric acid), the double bond of the alkene is broken, and a hydroxyl group is added to one carbon of the double bond, while a hydrogen atom is added to the other carbon. This forms an alcohol.
Explain the addition reaction of alkenes with potassium manganate (VII), in acidic conditions
This process is referred to as oxidation and produces a diol (compound containing two hydroxyl groups). Potassium manganate (VII) is the oxidising agent used in acid conditions. The reactions consists of oxidation followed by addition. During the reaction, potassium manganate (VII) provides one oxygen atom and water provides one oxygen atom and two hydrogen atoms. Potassium manganate VII changes from purple (MnO4- ion) to colourless (Mn2+ ion), so it’s another test for presence of alkene.
What is a heterolytic bond fission?
A heterolytic bond fission of a covalent bond results in the formation of a cation and anion as the two products produced: the atom that takes both electrons becomes an anion and the auto that loses both electrons becomes a cation.
Explain the mechanism of the electrophile addition reaction between alkenes and halogens
- Polarisation of the halogen molecule: the alkene reacts with the halogen molecule, causing the halogen molecule to become polarised, one atom becoming slightly positive (electrophile) and the other becoming slightly negative (nucleophile)
- Formation of the halonium ion: the slightly positive halogen atom reacts with the alkene to form a cyclic intermediate known as the halonium ion. This is a three-member end ring with the halogen atom bonded to both carbon atoms of the original double bond.
- Nucleophillic attack by halide ion: the halide ion attacks the more exposed carbon in the halonium ion, leading to the opening of the three membered ring and the formation of the final dihalogenated alkane.
Explain the mechanism of the electrophile addition reaction between alkenes and hydrogen halides
Attack by electrophile (H+) : the alkene has a pi bond which has high electron density, making it susceptible to attack by electrophiles. Here, the hydrogen halide is the electrophile, it dissociates to produce hydrogen ion and halogen ion. The pi bond in the alkene reacts with the H+ ion (electrophile) and the double bond breaks, forming a carbocation intermediate.
Formation of the carbocation intermediate: The carbocation forms at the carbon atoms of that is more substituted, as this provides more electron-donating effects, making the positive charge more stable.
Nucleophillic attack by the halide ion: once the carbocation forms, it’s highly electrophilic and can react with the halide ion which is the nucleophile. The halide ion attacks the carbocation, resulting in the formation of the final product - haloalkane.
Explain the mechanism of the electrophile addition reaction between an unsymmetrical alkene and a hydrogen halide
When the alkene is unsymmetrical (the two carbon atoms in the double bond aren’t the same), the addition of hydrogen halide follows Markovnikov’s rule. Which states:
- the hydrogen atom will add to the carbon atom of the double bond that already has the greater number of H atoms attached (less substituted carbon).
- The halide ion will add to the more substituted carbon (carbon with fewer H atoms)
Leading to the formation of the most stable carbocation intermediate at the more substituted carbon.
The stability of carbocations varies, state the order of carbocations from most stable to least:
Tertiary carbocation > secondary carbocation > primary carbocation/methyl carbocation
Why does the stability of carbocations vary due to its structure?
A carbocation in which the positive charge can be spread over more atoms is more stable than one in which there are fewer atoms available to spread the charge. Alkyl groups are electron-releasing, therefore the positive charge is spread more if there are more alkyl groups attached to the carbon. And so, the more stable the intermediate carbocation, the more easily it will be formed.
(Tertiary carbocations have more alkyl groups)
How should curly arrow notation be presented?
Curly arrows should start from either a bond or from a lone pair of electrons
What is the qualitative test for a C=C bond?
Using bromine water.
If the bromine water changes from orange/brown to colourless an alkene is present and so a C=C bond is present. If there’s no colour change, no C=C bond is present.
How can alkenes form polymers?
Through addition polymerisation
Waste polymers can be separated into specific types of polymers, what are these specific types of polymers used for?
- recycling
- incineration to release energy
- used as feedstock for cracking
Incineration is a way of dealing with polymer waste. How so?
Burning of polymers consists of hydrogen and carbon is a good way to produce heat to generate electricity. PVC (polyvinyl chloride) contains chlorine, and there are small amounts of toxic heavy metals from pigments used to colour plastics.
Using polymer waste as chemical feedstock is sometimes used to deal with the waste - what takes place during?
- Breaking down of polymer waste into gases (mainly H and CO)
- Similar process to cracking which breaks down the polymer chains into small molecules
- This produces feedstock for the manufacturing of new polymers and chemical reactions
What are pros and cons of using biodegradable polymers?
Pros
- Biodegradable polymers can be broken down into microbes so it’s renewable and better for environment
Cons
- They can be made from plant materials, meaning large amounts of land is needed to grow plants
- No useful substances can be extracted from the process of breaking down since all microbes go into environment directly
How have chemists limited the problems caused by polymer disposal by removing toxic gases caused by incineration of plastics?
- Ensuring complete combustion as incomplete combustion leads to unwanted, harmful products (CO and unburned hydrocarbons)
- Flue gas scrubbers which are systems that treat exhaust gases by passing them through an absorbent material or solution that captures toxic gases
- Use of catalysts helps break down harmful gases into less toxic substances
- Incineration in controlled environments
- Promoting recycling and use of biodegradable plastics
What are the three categories of halogenalkanes?
- Primary
- Secondary
- Tertiary
Define ‘nucleophile’
A nucleophile is an electron-rich species that can donate a pair of electrons - they are attracted to positively charged species’.
Describe the reaction of a halogenoalkane with aqueous potassium hydroxide
This reaction is Nucleophillic substitution of haloalkanes by KOH. This is heating a haloalkane with aqueous potassium hydroxide under reflux. Here, the nucleophile is the OH-. The product is an alcohol.
What does ‘under reflux’ mean?
Reflux involves heating the chemical reaction for a specific amount of time, while continually cooling the vapour produced back into liquid form, using a condenser.
Describe the reaction of haloalkanes with aqueous silver nitrate in ethanol
The reaction of haloalkanes with aqueous silver nitrate in ethanol results in a Nucleophillic substitution reaction, where water acts as the nucleophile and displaces the halide ion, forming an alcohol. The precipitation of silver halide indicates the identity of the halide and is used to test presence of different halogen atoms. Ethanol is used as a polar solvent that dissolves the haloalkane. The reaction mechanism can be SN1 or SN2 depending on the structure of the haloalkane, tertiary haloalkanes favouring SN1 mechanism and primary haloalkanes favouring SN2 mechanisms.
Describe the reaction of haloalkanes with potassium cyanide (cyanide ion acts as nucleophile)
- Heating a haloalkane with potassium cyanide dissolves in ethanol under reflux
- Nucleophile is CN- ion
- Produces a nitrile (formula is RCN)
- One more carbon is added to the reactant which is a useful way of extending the carbon chain
Describe the reaction of haloalkanes with ammonia
This is a Nucleophillic substitution of haloalkanes by NH3 where the haloalkane is heated with ammonia solution in a sealed tube. We use a sealed tube because ammonia is a has which can escape from the apparatus, this is dangerous as ammonia is toxic. The nucleophile is ammonia. The product is an amine.
Describe the reaction of haloalkanes with ethanolic potassium hydroxide
This is an elimination reaction that produces alkenes. Hydroxide ion acts as a base, not a nucleophile, and removes the H+ from a carbon adjacent to the one bonded to the halogen. Leading to the formation of a double bond between two carbons, resulting in an alkene and a by-product being the halide ion.
What is the result of the experiment that contains a test tube containing silver nitrate dissolved in ethanol with a chloroalkane and water.
No precipitate
What is the result of the experiment that contains a test tube containing silver nitrate dissolved in ethanol with a bromoalkane and water.
Faint cream-coloured precipitate
What is the result of the experiment that contains a test tube containing silver nitrate dissolved in ethanol with a iodoalkane and water.
Thick pale yellow precipitate
Describe the results of the ‘comparing the rates of hydrolysis reactions’ for when the halogenoalkanes with same structure but different halogens
It’s expected that the most polar bond (1-chlorobutane) would be the fastest to be hydrolysed die to its greatest partial positive charge on carbon, so the attacking nucleophiles are stronger. However, another important factor to consider is the amount of energy required to break the C-X bond. C-I bond is the weakest, so it breaks most easily to form I- ions for the precipitation of AgI (s). C-F bond is very strong meaning fluoroalkanes are unreactive, so shouldn’t be used in hydrolysis experiments. This shows that the strength of bonds are indirectly proportional to the bond length.
Describe the results of the ‘comparing the rates of hydrolysis reactions’ for when the halogenoalkanes have the same halogens but with different structures
The tertiary halogenoalkanes are more rapidly hydrolysed than secondary and primary compounds because the tertiary carbocation is much more stable than primary and secondary carbocations as these alkyl groups help stabilise the positive charge on the carbocation. So, in tertiary haloalkanes, since the carbocation is more easily formed and stable, the overall reaction proceeds faster, whereas, secondary and primary haloalkanes are slower because the carbocation intermediate they form are less stable so the reaction takes longer.
Describe the trend of reactivity in primary, secondary and tertiary halogenoalkanes in nucleophillic substitution
Primary
- Least reactive in S1 mechanism. Primary carbocations are very unstable, making it difficult for the C-X bond to break and form the carbocation, so S1 substitution isn’t favoured. S2 mechanism is more likely, where the nucleophile attacks the carbon at the same time the halogen leaves, in one step. It’s favoured by strong nucleophiles.
Secondary
- Less stable than tertiary but more stable than primary. S1 mechanism can occur but is slower than tertiary as the carbocation is less stable. S2 can also occur as it occurs in one step, it’s favoured by strong nucleophiles.
Tertiary
- Most reactive in S1 mechanism. Highly stable due to the three alkyl groups attached to the positively charged carbon atom. The stability allows the C-X bond to break more easily, forming a carbocation intermediate in the S1 mechanism. S1 mechanism is favoured as the carbocation is stable and readily formed, speeding up the reaction.
Describe the trend of reactivity in primary, secondary and tertiary halogenoalkanes in elimination reactions
Primary
- Least reactive in E1 mechanism. Very slow to undergo E1 due to the instability of primary carbocation. E2 elimination is more likely to occur when a strong base is present.
Secondary
- Moderate reactivity in E1 and E2 mechanisms. Secondary haloalkanes can undergo E1 elimination, but the formation of secondary carbocation is less stable than tertiary one ,making it slower than tertiary, E2 elimination is possible, where the base removes a proton from slightly negative carbon while halogen leaves simultaneously, leading to formation of alkene. E2 mechanisms are favoured in presence of strong base and higher temperatures.
Tertiary
- favour E1 mechanism due to the formation of a stable carbocation. In E1, the C-X bond breaks heterolytically to form a carbocation, which is deprotonated by a base (like hydroxide) to form alkene.
Explain, in terms of bond enthalpy, the trend in reactivity of chloro-, bromo- and iodoalkanes
Bond strength
- The C-X bond decreases as we move from C-Cl to C-I, due to the increase of the atomic size of halogen atoms. As the atomic radius of the halogens increases, the bond length increases, which weakens the bond because the overlap between the carbon and halogen orbitals becomes less effective. This weaker bond makes it easier for the halogen to leave during nucleophillic substitution process, making the alkane more reactive.
Breaking the C-X bond
- The C-I bind is the weakest, requiring least energy to break, so iodoalkanes undergo Nucleophillic substitution most readily. The C-Br bond is weaker than C-Cl but stronger than C-I, so bromoalkanes are more reactive than chloroalkanes but less reactive than iodoalkanes. C-Cl bond is strongest so chloroalkanes are least reactive in nucleophillic substitution reactions.
Describe the mechanism of the Nucleophillic substitution reaction between primary haloalkanes and aqueous potassium hydroxide
This follows S2 nucleophillic substitution mechanism. This is favoured because primary haloalkanes can’t form stable carbocations, which are required for S1 reactions. The hydroxide ion acts as the nucleophile and attacks the primary carbon of the haloalkane. The reaction proceeds with a backside attack, leading to the formation of an alcohol and the halide ion leaving as it’s being substituted.
Explain the mechanism of the Nucleophillic substitution reactions between primary haloalkanes and ammonia
Primary haloalkanes undergo nucleophillic substitution with ammonia via S2 mechanism. The ammonia attacks the carbon attached to the halogen, resulting in the formation of primary amine (R-NH2) and the departure of the halide ion. This reaction is bimolecular, meaning it depends on the concentration of both the haloalkanes and the nucleophile (NH3).
What can alcohols be classed as?
Primary, secondary or tertiary
What are the products of combustion?
Carbon dioxide and water
Describe the reaction between alcohols with PCl5
This is a chlorination reaction by phosphorus pentachloride to produce chloroalkane, phosphorus oxychloride, hydrogen chloride (can’t be hydrochloric acid as no water is present. Room temperature condition (no heating is needed as the reaction is vigorous).
How can you convert tertiary alcohols into halogenalkanes?
By the chlorination of tertiary alcohols by concentrated hydrochloric acid. Condition: room temperature. This produces chloroalkanes and water. This method doesn’t work well for primary and secondary alcohols as they don’t have fully stable carbocation making the reaction very slow.
Describe the reaction between alcohols and 50% concentrated sulfuric acid and potassium bromide
This reaction is bromination by mixture of potassium bromide and 50% concentrated sulfuric acid to produce bromoalkane and water. The conditions required is warm.
Describe the reaction between alcohols with red phosphorus and iodine
This reaction is the iodinating by mixture of red phosphorus and iodine. The conditions required is to be heated under reflux. This produces iodoalkanes and phosphoric acid.
Describe the reaction of primary alcohols with potassium dichromate (VI) in dilute sulfuric acid
Primary alcohols are oxidised to aldehydes first, and with further oxidation, to carboxylic acids. The potassium dichromate in dilute sulfuric acid acts as oxidising agent. The dichromate ion is reduced to chromium (III) ion, while the alcohol is oxidised. If the oxidation is continued or the reaction conditions are stronger (excess oxidant or heat), the aldehyde can be further oxidised to a carboxylic acid.
Describe the test for an aldehyde
- Add Benedict’s solution or Fehling’s solution
- Both include Cu (II) ions, which are reduced to Cu (I) ions in the presence of an aldehyde group, forming a brick-red precipitate of copper oxide.
Describe the reaction of a secondary alcohols with potassium dichromate (VI) in dilute sulfuric acid
Secondary alcohols are oxidised to ketones when reacted with potassium dichromate (VI) in dilute sulfuric acid. The oxidising agent (dichromate ion) will remove two hydrogen atoms from the secondary alcohols, forming a ketone. This can be identified by a lack of change when using Benedict’s or Fehling’s solutions as these are specific tests for aldehydes.
Describe the reaction of alcohols with concentrated phosphoric acid
This reaction is an elimination reaction which produces alkenes by dehydration. When a water molecule is eliminated from an alcohol, the alcohol is converted to an alkene, this is dehydration. Dehydration is a chemical change in which hydrogen and oxygen are eliminated in a ratio of 2:1 from a compound. A OH group and a hydrogen atom from an adjacent carbon atom being removed causes a C=C bond to form in the chain. This is carried out at an elevated temperature in the presence of a dehydrating agent (concentrated phosphoric acid).
How is simple distillation used to purify an organic liquid?
Distillation of an impure liquid involves heating it in a flask connected to a condenser. The liquid with the lowest boiling point evaporates or boils first and passes into the condenser first. Meaning it can be collected in the receiver separately from any other liquid that evaporates later. The purpose of the thermometer is to monitor the temperature of the vapour as it passes into the condenser. If the temperature remains steady, it’s an indication that one compound remains distilling over. If, after a while, the temperature begins to rise, this indicates that a different compound is distilling over.
How is fractional distillation used to purify an organic liquid?
Fractional distillation uses the same apparatus as simple distillation, but with a fractionating column between the heating flask and the still head. The column is usually filled with glass beads or pieces of broken glass, which acts as surfaces on which vapour leaving the column can condense and then be evaporated again as more hit vapour passes up the column. Effectively, the vapour undergoes several repeated distillations as it passes up the column, which provides a better separation. Fractional distillation takes longer than simple distillation, and is best used when the difference in boiling temperatures is small, and when there are several compounds to be separated from the mixture.
How is solvent extraction used to purify organic liquids?
Solvent extraction involves using a solvent to remove the desired organic product from the other substances in the reaction mixture. There are several solvents that can be used, but the choice depends mainly on these features: the solvent added should be immiscible (not mixable) with the solvent containing the desired organic product and the desired organic product should be much more soluble in the solvent added in the reaction mixture. If a suitable solvent is used and method is followed correctly, most of the desired organic product will have moved into the added solvent. It’s better to use the solvent in small portions rather than in a single large volume as it’s more efficient. Using more portions of solvent but with the same total volume, removes more of the desired organic product. The desired organic product has been removed from the reaction mixture, but is now mixed with the added solvent. So, simple distillation or fractional distillation now has to be used to separate the desired organic product from the solvent used.
How is drying used to purify organic liquids?
Many organic liquids are prepared using inorganic reagents, which are often used in aqueous solutions. A liquid organic product may partially or even completely dissolve in water, so water may be an impurity that needs to be removed by a drying agent. One important feature of a drying agent is that it does not react with the organic liquid.
There are several drying agents available, but the most common ones are anhydrous metal salts, often anhydrous calcium sulfate, magnesium sulfate and sodium sulfate. What these compounds have in common is that they form hydrated salts, so when they come into contact with water in an organic liquid they absorb the water as water of crystallisation. Anhydrous calcium chloride can also be used for some organic compounds, although it does react with others and is soluble in alcohols. Before use, a drying agent is powdery, but after absorbing water it looks more crystalline.
If a bit more drying agent is added, and it remains powdery, this is an indication that the liquid is dry. The drying agent is removed either by decantation (pouring the organic liquid off the solid drying agent), or by filtration.
How can you test if organic liquids are pure?
For liquids, there is a simple way to test whether it is pure - measure its boiling temperature. Impurities raise the boiling temperature.
The boiling temperatures of pure organic compounds have been carefully measured and are widely available in data books and online. If you measure the boiling temperature of your organic compound, you can compare it with an accurate value and then make your decision about how pure it is.
Different organic compounds can, by coincidence, have the same boiling temperature. For example, both 1-chloropentane and 2-methylpropan-1-ol boil at 108oC.
Why can incomplete combustion occur?
Limited supply of oxygen
Why oil companies reform alkanes like heptane?
- higher octane number
- more efficient combustion
- allows smoother burning
- reduces knocking
What condition is essential for a reaction of an alkane with a halogen?
UV light
Write the two propagation steps for the mechanism of chlorine and butane
- Cl (with radical) + C4H10 —> C4H9 (with radical) + HCl
- C4H9 (with radical) + Cl2 —> C4H9Cl + 2Cl (with radical)
Why does but-1-ene not exhibit E-Z isomerism?
C on one end of the double bond is not attached to two different atoms or groups
Why is ethanol used in a reaction using Chlorine?
Ethanol is a cosolvent and so it dissolves halogenoalkanes
A precipitate forms as a result of reactions between aqueous silver ions and aqueous halide ions. Explain why halide ions are present in the mixture containing a haloalkane which only has covalent bonds.
The halogenoalkane is hydrolysed by water. So, the C-Halogen bond breaks heterolytically
