Topic 6: Organics 1 Flashcards
This covers key definitions, Alkanes, Alkenes, Halogenoalkanes, Alcohols and Mechanisms
What is a Hydrocarbon?
A compound exclusively consisting of hydrogen and carbon atoms.
What is a Homologous series?
Series of organic compounds with the same functional group and general formula. Consecutive members of a series differ by -CH2 .
What is Polymerisation?
Joining together lots of simple molecules (monomers) to form a giant molecule (a polymer).
What is a Functional group?
a group of atoms responsible for the characteristic reactions of a particular
compound.
What is an Elimination reaction?
When a small group of atoms breaks away from a larger molecule to form a C=C bond.
What is a Substitution reaction?
When one species is replaced by another.
What is Heterolytic fission?
The process of breaking a covalent within a molecule leading to the formation of ions. Upon bond breaking, one atom receives the electron pair and becomes a negatively charged ion. Other atom becomes a cation, e.g.
Br-Br → Br + + Br −
What is Homolytic fission?
The process of breaking a covalent within a molecule leading to the formation of radicals. Upon bond breaking, each atom receives one electron from the bonding pair and both atoms become radicals, e.g.
Br-Br → Br ⋅ + Br ⋅
What is a Radical?
A species with an unpaired electron. Represented in mechanisms by a single dot.
What is Free radical substitution?
Photochemical reaction (requires UV light) between halogens and alkanes to form halogenoalkanes. Initiation is the process of forming the radicals from a molecule by homolytic fission. Propagation is the formation of a new radical and a new molecule from some radical and other molecule. Termination concludes the mechanism and is the process of two radicals joining together to form a molecule. Polysubstitution (multiple substitution) is often a problem.
Stereoisomerism
Occurs when two double bonded carbon atoms each have two different atoms or groups attached to them. Includes E /Z isomerism and cis/trans isomerism. This is a consequence of a restricted rotation around the C=C double bond.
Structural isomerism
Occurs when species have the same molecular formula, but a different structural formula, e.g. C6H12 can be ascribed to hex-1-ene, but also to 2-methylpent-1-ene.
Cracking
Breaking long chain alkanes into smaller, more useful hydrocarbons. Helps to convert low demand hydrocarbons into more highly demanded ones.
Reforming
Processing of straight chain hydrocarbons into branched chain alkanes and cyclic hydrocarbons for efficient combustion. Done so there’s no knocking.
Knocking
Alkanes explode of their own accord when the fuel/air mixture in an engine is compressed.
Complete combustion
Produces fully oxidised products (e.g. CO2) as opposed to incomplete
combustion (produces CO).
Catalytic Converters
Get rid of pollutants in cars by using platinum catalyst to convert them to harmless gases, e.g. 2NO + CO → N2+ CO2.
Biofuels
Fuels made from living matter over a short period of time. e.g. biodiesel made by refining renewable fats and oils such as vegetable oil.
Feedstock
For conversion of polymer waste into compounds that can be used to synthesise new polymers.
Nucleophile
Electron pair donors in an organic mechanism. Attracted to electron-deficient areas.
Electrophile
Electron pair acceptor in an organic mechanism. Attracted to areas with lot of electrons/high negative charge.
Markovnikov’s rule
When a compound HX is added to an unsymmetrical alkene, the hydrogen becomes attached to the carbon with the most hydrogens attached to it already.
The major product of an electrophilic addition to the unsymmetrical alkene results from a reaction proceeding via the most stable carbocationic intermediate (stability increases
in the order: primary<secondary<tertiary).
Carbocation
A carbon atom bearing a positive charge.
Distillation with addition
Performing the reaction under distillation conditions whilst adding one of the reagents. The product distills off as it forms in case of oxidation of an alcohol to an aldehyde.
Fractional distillation
A distillation that utilises a fractionating column (packed glass beads that provide a surface for the vapour to condense and evaporate again). Used to separate liquids of
similar boiling points.
Solvent extraction
A method for separating a compound from a mixture by causing it to move to another solvent.
State the reagent, conditions, main reactant and products of Chlorination
Reagent: PCl5 (or PCl3)
Conditions: Room temp.
Reactant: Alcohol
Product: Chloroalkane, POCl3 + HCl (seen as misty fumes)
What is chlorination used as a test for?
Tests for alcohols –> misty fumes given off
All alcohols react with PCl5
State the reagent, conditions, main reactant and products of Bromination
Reagent: NaBr in concentrated H2SO4
Conditions: Heat and distil
Reactant: Alcohol + HBr
Product: Bromoalkane + water
What do you need to remember for Bromination?
The HBr is made in situ:
NaBr + H2SO4 –> HBr
State the reagent, conditions, main reactant and products of Iodination
Reagent: NaI in concentrated H3PO4 (or PI3)
Conditions: Heat and distil
Reactant: Alcohol + PI3
Product: Iodoalkane + H3PO3
How do we make the PI3 required for Iodination?
Made in situ, (as unstable) reaction between Red Phosphorus and Iodine. Heat under reflux
2P + 3I2 –> 2PI3
What causes reactions with halogenoalkanes?
The polarity of the C-X bond, which leaves the carbon slightly electron deficient so vulnerable to attack by nucleophiles.
State the reagent, conditions, main reactant and products of Hydrolysis with water
Substitution reaction:
Reagent: Water
Conditions: Room temp
Reactant: Halogenoalkane + water
Product: Alcohol + H+ + X- (if water used)
Why is ethanol used in Hydrolysis?
It is a mutual solvent, as halogenoalkanes are not soluble in water.
How can you compare rates of Hydrolysis of different halogenoalkanes?
Addition of AgNO3.
Time how long it takes each different halogenoalkane to turn cloudy, as silver Halide precipitate is formed.
Why are OH- ions better nucleophiles than water molecules?
The hydroxide ion is a better nucleophile than water as it carries a full negative charge
In water, the oxygen atom only carries a partial charge.
Hence, reactions with water are slower.
State the reagent, conditions, main reactant and products of Hydrolysis with hydroxide ions.
Reagent: NaOH or KOH
Conditions: Room temp (or slightly warmed)
Reactant: Halogenoalkane + OH-
Product: Alcohol + Hydrogen halide
State the reagent, conditions, main reactant and products of reaction with cyanide ions.
Substitution reaction:
Reagent: KCN in ethanol
Conditions: Heat under reflux
Reactant: Halogenoalkane + CN-
Products: Nitrile + halide ion
Why is substitution with cyanide ions helpful?
Addition of the cyanide ion extends the carbon chain by adding an extra carbon atom
Example of substitution with cyanide ions?
Bromoethane is heated under reflux with ethanolic potassium cyanide to form propanenitrile (and a bromide ion)
State the reagent, conditions, main reactant and products of reaction with ammonia?
Substitution reaction:
Reagent: concentrated NH3 in ethanol
Conditions: Sealed and heated under pressure
Reactant: Halogenoalkane + NH3
Products: Primary amine (NH2 group on end of alkane) + Hydrogen halide
Why is excess ammonia used in substitution with ammonia?
Because the product is more reactive than ammonia so further substitution reactions could occur
Example of substitution with ammonia?
bromoethane reacts with excess ethanolic ammonia when heated under pressure to form ethylamine and hydrogen bromide
State the reagent, conditions, main reactant and products of elimination reactions.
Reagent: Ethanolic KOH or NaOH
Conditions: Heat
Reactant: Halogenoalkane + NaOH
Products: Alkene + water + NaX (or KX)
Example of elimination reaction of halogenoalkane?
The halogenoalkanes are heated with ethanolic sodium hydroxide causing the C-X bond to break heterolytically, forming an X- ion and leaving an alkene as an organic product
E.g. bromoethane is heated with ethanolic sodium hydroxide to form ethene, water + Sodium bromide
What are the stages of Free radical substitution?
Initiation
Propagation
Termination
What happens in the initiation stage of FRS?
Ultraviolet light breaks the halogen molecule into two atoms.
Each halogen atom gains one electron from the bond, forming a radical
This is homolytic fission
Cl–Cl —> Cl. + Cl.
What happens in the Propagation stage of FRS?
The radical reacts with the other molecule to form another radical and Hydrogen halide.
e.g.
Cl . + CH4 –> HCl + CH3 .
the radical formed then goes on to react again:
CH3. + Cl2 –> Ch3Cl + Cl.
What happens during the termination stage of FRS?
Each type of radical will react together:
Cl. + Cl. –> Cl2
Cl. + CH3. –> Ch3Cl
CH3. + CH3. –> C2H6
Compare the physical properties of alcohols and alkanes of similar Mr
Alcohols are much less volatile than alkanes of similar molar mass
This is due to hydrogen bonding between -OH groups in alcohols, which is much stronger than London forces between alkanes
Can alcohols mix with water?
Yes - short chain alcohols can mix with water. This is due to hydrogen bonding between the H of water and O of the alcohol.
Are alcohols more or less reactive than alkanes?
More reactive due to the C-O and O-H bonds being polar
Why can’t long chain alcohols mix with water?
The longer the chain, the more H-bonds between water molecules need to be broken. Because similar forces of attraction do not occur between the alcohol and water molecules, they don’t dissolve.
What are the reagents and conditions required for oxidation of a primary alcohol?
Acidified potassium dichromate and dilute sulfuric acid.
In equations, the oxidising agent is written as [O]
What happens when a primary alcohol is oxidised?
It will first be oxidised to an aldehyde. However, because aldehydes are more easily oxidised than alcohols, the aldehyde will be further oxidised to form a carboxylic acid.
E.g. Propanol + [O] –> Propanal (aldehyde)
Propanal + [O] –> Propanoic acid (carboxylic acid)
What happens when a secondary alcohol is oxidised?
It will form a ketone.
E.g. Propan-2ol + [O] —> Propanone
Ketones cannot be further oxidised.
What happens when an aldehyde is oxidised?
It will gain an oxygen atom, which goes between the C and H of the CHO group.
How does distillation with addition work?
Only the oxidising agent is heated.
The alcohol is slowly added, and when the aldehyde is formed, it immediately distils off and is collected in the receiver.
(aldehydes have a lower boiling point than alcohol)
What does the dehydration of an alcohol form?
Alkenes.
Why do we heat alcohols under reflux?
To obtain a ketone or carboxylic acid.
Oxidation is complete.
Why do alcohols undergo distillation with addition?
Oxidation is intended to be incomplete.
Obtain aldehyde, not carboxylic acid.
What are the reagents and conditions required for dehydration of alcohols?
Heat with concentrated phosphoric acid.
the OH group and H atom from adjacent carbon atoms are removed and a C=C double bond formed in the middle of the chain.
Forms alkenes
