Alcohols and Haloalkanes (Chapter 14 and 15) Flashcards
What is an alcohol?
An organic compound containing the functional group OH (R-OH)
What is the name for the OH group?
Hydroxyl group
When does the name of the alcohol have the suffix “ol”?
When it is the highest priority functional group in a molecule
When does the name of the alcohol have the prefix “hydroxy”?
When it is not the highest priority functional group in a molecule
When is an alcohol primary?
When the carbon atom adjacent (attached) to the OH group is bonded to two hydrogen atoms
When is an alcohol secondary?
When the carbon atom adjacent (attached) to the OH group is bonded to one hydrogen atom
When is an alcohol tertiary?
When the carbon atom adjacent (attached) to the OH group is not bonded to hydrogen
How do the physical properties of alcohols compare to those of corresponding alkanes? (alcohol vs alkane with same number of carbons)
Less volatile
Higher melting points
Greater water solubility
How do the differences between the physical properties of alcohols change as the carbon chain becomes longer?
The differences become much smaller as the length of the carbon chain increases
Explain the differences between the physical properties of alkanes and alcohols (considering bond polarity and the effects on the intermolecular forces)
- The alkanes have non-polar bonds because the electronegativity of H and C are very similar
- The alkane molecules are therefore non-polar
- The intermolecular forces between non-polar molecules are very weak London forces
- Alcohols have a polar OH bond because of the difference in electronegativity between the O and H atoms
- Alcohol molecules are therefore polar
- The intermolecular forces will be very weak London forces but there will also be much stronger hydrogen bonds between the polar OH groups
Why do alcohols have a lower volatility and higher boiling point than alkanes?
- In the liquid state, H bonds hold the alcohol molecules together
- These bonds must be broken in order to change the liquid alcohol into a gas
- This requires more energy than overcoming the weaker London forces in alkanes, so alcohols have a lower volatility than the alkanes with the same number of carbon atoms
What makes a compound much more soluble in water than other compounds?
If it can form H bonds with water
Why are certain alcohols soluble in water and alkanes are not?
- Alkanes are non-polar molecules and cannot form H bonds with water
- Alcohols such as methanol and ethanol are completely soluble in water, as hydrogen bonds form between the polar OH group of the alcohol and the water molecules
- The energy gained from H bonding with water is enough to break the London forces between alcohol molecules
Why does solubility of alcohols decrease as the length of the hydrocarbon chain increases?
- As the hydrocarbon chain increases in size, the influence of the OH group becomes relatively smaller, and the solubility of the longer-chain alcohols becomes more like that of hydrocarbons, so solubility decreases
- The energy gained from H bonding with water is not enough to break the larger number of London forces between alcohol molecules
Describe the combustion of alcohols
- Alcohols burn completely in a plentiful supply of oxygen to produce carbon dioxide and water
- The reaction is exothermic, releasing a large quantity of energy in the form of heat
- As the number of carbon atoms in the alcohol chain increases, the quantity of heat released per mole increases
How can alcohols be oxidised?
- Primary and secondary alcohols can be oxidised by an oxidising agent
What is the usual oxidising mixture used to oxidise alcohols?
A solution of potassium dichromate (VI) (K2Cr2O7), acidified with dilute sulfuric acid (H2SO4)
How can you tell when an alcohol has been oxidised?
The orange solution containing dichromate (VI) ions is reduced to a green solution containing chromium (III) ions
What are the two possible products of the oxidation of a primary alcohol?
Aldehyde or carboxylic acid
What does the product of oxidation of primary alcohols depend on?
The reaction conditions (distillation/reflux) used because aldehydes are themselves also oxidised to carboxylic acids
Describe the preparation of aldehydes from the oxidation of primary alcohols
- On gentle heating of primary alcohols with acidified potassium dichromate, an aldehyde is formed
- To ensure that the aldehyde is prepared rather than the carboxylic acid, the aldehyde is distilled out of the reaction mixture as it forms
- This prevents any further reaction with the oxidising agent
- The dichromate ions change from orange to green
What is the symbol used to represent the oxidising agent?
[O]
Why do aldehydes have a lower BP than alcohols?
Because they have weaker H-bonding
Describe the preparation of carboxylic acids from the oxidation of primary alcohols
- If a primary alcohol is heated strongly under reflux, with an excess of acidified potassium dichromate (VI), a carboxylic acid is formed
- Use of an excess of the acidified potassium dichromate (VI) ensures that all of the alcohol is oxidised
- Heating under reflux ensures that any aldehyde formed initially in the reaction also undergoes oxidation to the carboxylic acid
- Once the reaction has gone to completion, the carboxylic acid can then be distilled or recrystallised
What is important to remember about the oxidation producing carboxylic acid equation?
That the ratio of the alcohol to the oxidising agent is 1:2 - 2[O]
(summary) How do you prepare a carboxylic acid?
Heat a primary alcohol under reflux
(summary) How do you prepare an aldehyde?
Use distillation to remove the aldehyde from the reaction mixture
What is the product of oxidising a secondary alcohol?
Ketone
Is it possible to further oxidise ketones?
Not with acidified dichromate (VI) ions
Describe the preparation of ketones from the oxidation of secondary alcohols
- To ensure the reaction goes to completion, the secondary alcohol is heated under reflux with the oxidising mixture
- The dichromate (VI) ions once again change colour from orange to green
Can tertiary alcohols undergo oxidation reactions?
No, the acidified dichromate (VI) remains orange when added to a tertiary alcohol
What is a dehydration reaction?
Any reaction in which a water molecule is removed from the starting material
Describe the dehydration reaction of an alcohol
- An alcohol is heated under reflux in the presence of an acid catalyst such as concentrated sulfuric acid (H2SO4) or concentrated phosphoric acid (H3PO4)
- The product of the reaction is an alkene (and water)
What is dehydration of an alcohol an example of?
An elimination reaction
What is the example of a substitution reaction of alcohols?
Alcohols react with hydrogen halides to form haloalkanes
How do you prepare a haloalkane?
- The alcohol is heated under reflux with sulfuric acid and a sodium halide
- The hydrogen halide is formed in situ
- The hydrogen halide formed reacts with the alcohol to produce the haloalkane
Give an example equation for how the hydrogen halide is formed
NaBr(s) + H2SO4(aq) = NaHSO4(aq) + HBr(aq)
What is the overall equation for the reaction of propan-2-ol with H2SO4 and NaBr?
CH3CHOHCH3 + NaBr + H2SO4 = CH3CHBrCH3 + NaHSO4 + H2O
What is an aldehyde?
C bonded with R, H and double bond O
What is a carboxylic acid?
C bonded with R, OH and double bond O
What is a ketone?
C bonded with R1, R2 and double bond O
What is the suffix for an aldehyde?
“al”
What is the suffix for a ketone?
“one”
What is the prefix for a ketone?
“oxo”
What is the suffix for a carboxylic acid?
“oic acid”
What are haloalkanes?
Compounds containing the elements carbon, hydrogen and at least one halogen
How do you name haloalkanes?
- A prefix is added to the name of the longest chain to indicate the identity of the halogen
- When two or more halogens are present in a structure they are listed in alphabetical order
Why is the carbon-halogen bond in a haloalkane polar?
- Haloalkanes have a carbon-halogen bond in their structure
- Halogen atoms are more electronegative than carbon atoms
- The electron pair in the carbon-halogen bond is therefore closer to the halogen atom than the carbon atom
- The carbon-halogen bond is polar
Why can carbon atoms in haloalkanes attract nucleophiles?
Because it has a slightly positive charge due to the polarity of the carbon-halogen bond
What is nucleophile?
- Species that donate a lone pair of electrons
- An atom of group of atoms that is attracted to an electron deficient carbon atom, where it donates a pair of electrons to form a new covalent bond
Give 3 examples of nucleophiles
Hydroxide ions - :OH-
Water molecules - H2O:
Ammonia molecules - :NH3
What happens when a haloalkane reacts with a nucleophile?
- The nucleophile replaces the halogen in a substitution reaction
- A new compound is produced containing a different functional group
What is the reaction mechanism for when a haloalkane reacts with a nucleophile?
Nucleophilic substitution (heterolytic fission)
What is substitution?
A reaction in which one atom or group of atoms is replaced by another atom or group of atoms
Which haloalkanes undergo nucleophilic substitution?
Primary haloalkanes
What is hydrolysis?
- A chemical reaction involving water or a aqueous solution of a hydroxide that causes the breaking of a bond in a molecule
- This results in the molecule being split into two products
What happens in the hydrolysis of a haloalkane?
The halogen atom is replaced by an OH group (e.g. in an aqueous alkali), forming an alcohol
Describe the hydrolysis of a haloalkane
- The nucleophile, OH-, approaches the carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom (minimising repulsion between the nucleophile and the d- halogen atom)
- A lone pair of electrons on the hydroxide ion is attracted and donated to the d+ carbon atom
- A new bond is formed between the oxygen atom of the hydroxide ion and the carbon atom
- The carbon-halogen bond breaks by heterolytic fission
- The new organic product is an alcohol - a halide ion is also formed
Describe the reaction between haloalkanes and aqueous sodium hydroxide
- Haloalkanes can be converted to alcohols using aqueous sodium hydroxide
- The reaction is very slow at room temperature, so the mixture is heated under reflux to obtain a good yield of product
- NaBr is also formed
What happens to the bonds during hydrolysis of a haloalkane?
The carbon-halogen bond is broken and the OH group replaces the halogen in the haloalkane
What does the rate of hydrolysis of a haloalkane depend on?
The strength of the carbon-halogen bond in the haloalkane
Which carbon-halogen bond is the strongest and which is the weakest and what does this mean?
C-F bond is the strongest and C-I bond is the weakest (bc C-I is a longer bond)
This means that less energy is required to break the C-I bond than other carbon-halogen bonds
What can we predict from the bond enthalpies of the carbon-halogen bonds?
- Iodoalkanes react faster than bromoalkanes
- Bromoalkanes react faster than chloroalkanes
- Fluroroalkanes are unreactive as a large quantity of energy is required to break the C-F bond
What is the general equation for the hydrolysis of halobutanes with water? (X representing any of the halogens)
CH3(CH2)3X + H2O = CH3(CH2)3OH + H+ + X-
How can you measure the rate of hydrolysis of primary haloalkanes (halobutanes)?
- The rate of each reaction can be followed by carrying out the reaction in the presence of aqueous silver nitrate
- As the reaction takes place, the halide ions, X-(aq), are produced which react with Ag+(aq) ions to form a precipitate of the silver halide
- Observe the test tubes for five minutes and record the time taken for the precipitate to form
Why is the hydrolysis reaction of primary haloalkanes (with AgNO3) carried out in the presence of an ethanol solvent?
- The nucleophile in the reaction is water, which is present in the aqueous silver nitrate
- Haloalkanes are insoluble in water
- The ethanol allows water and the haloalkane to mix and produce a single solution rather than two layers
What are the observations that should be found during the hydrolysis of haloalkanes with AgNO3?
Iodo - yellow precipitate forms rapidly
Bromo - cream precipitate forms slower
Chloro - white precipitate forms very slowly
How can the observations be explained in the reaction of the haloalkane with AgNO3 and what conclusions can be drawn?
- The compound with the slowest rate of reaction is he one that has the strongest carbon-halogen bond
- Therefore, the rate of hydrolysis increases as the strength of the carbon-halogen bond decreases
What are some uses of organohalogen compounds?
Pesticides, general solvents (CHCl3), dry cleaning solvents (C2HCl3), making polymers (C2F4), flame retardants (CF3Br) and refrigerants (F2CCl2)
What is the ozone layer?
- The ozone layer is found 10-40km above the Earth’s surface, at the outer edge of the stratosphere.
- Ozone absorbs UV-B radiation from the Sun that causes sunburn, genetic damage and skin cancer
What reactions are occurring in the ozone layer naturally?
- Ozone is constantly being formed and broken down by the action of UV radiation:
O2 = 2O (oxygen radicals) - Ozone is then formed with oxygen and oxygen radicals:
O2 + O <=> O3
How do you draw an oxygen radical?
O (no dot)
Why are CFCs so stable?
- Due to the strength of their carbon-halogen bonds
What happens once CFCs enter the stratosphere?
- UV radiation provides sufficient energy ti break a carbon-halogen bond in CFCs by homolytic fission to form halogen radicals
- The C-Cl bond has the lowest bond enthalpy and so is the bond that breaks
What is the name of he process as radiation initiates the breakdown of CFCs?
Photodissociation
What equation takes place during photodissociation?
CF2Cl2 = CF2Cl• + Cl•
What happens during propagation in the breakdown of CFCs?
The chlorine radical formed, Cl•, is a very reactive intermediate and can react with an ozone molecule, breaking down the ozone into oxygen
What two equations take place during propagation of CFCs?
Step 1: Cl• + O3 = ClO• + O2
Step 2: ClO• + O = Cl• + O2
What is the overall equation of propagation of CFCs (or other radicals e.g. NO•) that shows how they breakdown ozone?
O3 + O = 2O2
Why can the cycle of propagation of CFCs happen many times (up to 100,000)?
- Step 2 regenerates a chlorine radical, which can attack and remove another molecule of ozone in step 1
- The two propagation steps repeat in a cycle over and over again in a chain reaction
- It is estimated that a single CFC molecule can promote the breakdown of 100,000 molecules of ozone
What other radicals can catalyse the breakdown of ozone?
Nitrogen oxide radicals (NO•) which are formed naturally during lightening strikes and also as a result of aircraft travel in the stratosphere
What are the two equations in the propagation steps showing the breakdown of ozone by NO•?
Step 1: NO• + O3 = NO2• + O2
Step 2: NO2• + O = NO• + O2
What do you need to remember when writing oxidation reactions of alcohols?
WATER IS PRODUCED
What do you have to remember about the dehydration of alcohols to alkenes?
You can’t form a branched alkene from a straight chain alcohol
What three things do you need to draw on a diagram showing H-bonding?
- Dipoles
- Lone pair
- Hydrogen bond
What does low volatility mean?
- Requires a high amount of energy to turn into a gas
- Is not easily vaporised
What reasons are there to use CFC’s (or related compounds) as aerosols?
- Non toxic, high volatility, low flammability, low reactivity, will not hydrolyse in water, low BP
Why might a CFC not hydrolyse in water?
Because the C-F bond is very strong
Why might you need to redistill an organic product?
To get rid of any impurities e.g. organic side product or unreacted alcohol
Why do you add an acid catalyst slowly?
To avoid a big temperature increase which avoids side-reactions
