5- Alcohols

What is a primary, secondary and tertiary alcohol?

Primary - one carbon attached

Secondary - two carbons attached

Tertiary - three carbons attached

Solubility of alcohols

Generally soluble, the larger the hydrocarbon chain attached to the OH group, the less soluble in water.

The hydrogen on the hydroxyl group bonds with the lone pair on the oxygen of water this is why its soluble

Why do alcohols have low volatility? (doesn't evaporate readily)

Strong hydrogen bonding within the alcohol, less likely to break and hence evaporate

Boiling points of alcohols

The larger the Mr (hydrocarbon chain) the higher the boiling point as there are larger van De Waals forces

Uses of alcohols

-Chemical feedstocks

-Disinfectants

-Drinks

-Antifreeze

Shape of alcohols

C-O-H angle 104.5 since there are two lone pairs

Why are shorter hydrocarbon chains soluble in water but longer hydrocarbon chains are not

The OH- group of alcohols can hydrogen bond to water molecules, but the non polar hydrogen chain cannot

Shorter chains - soluble in water as the hydrogen bonding predominates

Longer chains - insoluble as the non-polar hydrocarbon chain predominates

Outline the mechanism for the formation of an alcohol by the reaction of an alkene with steam in the presence of an acid catalyst, use ethene as an example. State the conditions for this reaction

H3PO4 catalyst, high temp (300 degrees) pressure 60atm

a) Dehydration of alcohols

b) How can you make ethene from ethanol

a) CNH2N+1OH ---> CNH2N + H20

You can make alkenes by eliminating water from alcohols

b) Heat ethanol with a H2S04/H3PO4 catalyst

The mechanism for this reaction is the opposite of the mechanism for the formation of an alcohol

The positively charged oxygen pulls electrons away from the carbon, h2o molecule leaves

Method to get a pure alkene initially produced from a dehydration reaction, stage 1

Distillation - uses the fact that different chemicals have different boiling points to separate them

For example here's how to produce cyclohexene from cyclohexanol

STAGE ONE: distillation

1) Add conc H2SO4 and H3PO4 to a flask containing cyclohexanol. Mix.

2) Connect flask to rest of distillation apparatus, including thermometer condenser and cooled collection flask

3) Gently heat the mixture to the boiling point of cyclohexene w water bath (chemicals w boiling points lower will evaporate and rise out of flask to condenser)

4) At condensor cooler temps turn the gases back into liquids

5) Liquid product collected

Method to get a pure alkene initially produced from a dehydration reaction, stage 2

Seperation phase - product collected after first stage will still have some impurities

1) Transfer product mixture to separating funnel, add water to dissolve solid impurities --> creates an aqueous solution

2) Allow mixture to separate into layers, drain off aq layer

Method to get a pure alkene initially produced from a dehydration reaction, stage 3

Purification phase

1) Drain off impure cyclohexene into flask

2) Add CaCl2 (drying agent) to flask

3) Distill one more time and the product produced at cyclohexene boiling point will be the pure mixture itself

Industrial production of ethanol by fermentation

Fermentation is an exothermic process

Conditions: 30-40 degrees C, anaerobic conditions

Yeast produces enzymes which concert glucose into ethanol and carbon dioxide

C6H12O6 ---> 2C2H5OH + 2CO2

Fermentation v Hydration of ethene to produce ethanol

Fermentation:

-slow

-impure, needs further processing

-sugars are a renewable energy source

-batch process, cheap but high labour costs

-produces CO2

Hydration of ethene:

-fast

-pure

-ethene from crude oil, finite resource

-continuous process, expensive equipment needed, but low labour costs

-Pollution of hydrocarbons escape

What is meant by a biofuel?

Fuel that is made from biological material that has recently died such as sugars from sugarcane can be fermented to produce ethanol

Ethanol from fermentation is a biofuel

Advantages and disadvantages of biofuels

Advantages:

-Renewable energy resource

-Produces CO2 when burnt but burning a biofuel only releases the same amount of CO2 that the crop plant took in as it was growing, carbon neutral

Disadvantages:

-Occupies land that could have been used to grow food, unable to feed everyone (food v fuel debate)

-Deforestation to create land for biofuels

-Fertilisers used to increase biofuel crop production, pollute waterways, some fertilised produce NOX

-Current car engine would be unable to run on fuels with high concentrations of ethanol

Equations that prove biofuels are carbon neutral

1) Photosynthesis of plants

6CO2 + 6H2O ---> C6H12O6 + 6O2

2) Fermentation, glucose into ethanol

C6H12O6 ---> 2C2H5OH + 2CO2

3) Ethanol is burned

2C2H5OH + 602 ---> 4CO2 + 6H20

If you combine the three equations, exactly 6 mols of CO2 are taken in and also given out

Why might biofuels not be considered carbon neutral

-Fossil fuels will be needed to be burned to power the machinery used to make fertilisers for the crops and machinery used to harvest the crops

-Refining/transporting bioethanol uses energy

Can tertiary alcohols be

a) oxidised

b) dehydrated

a) no

b) yes

What are primary and secondary alcohols oxidised to

Primary - to aldehydes then further oxidised to carboxylic acids

Secondary - to ketones

How can we maximise our yield when oxidising an alcohol through distillation

Vapours are truly condensed, place boiling tube holding product in an ice bath

Method of oxidising an alcohol to make a carboxylic acid straight away

Reflux

a) Ethanol oxidised into ethanal equation

b) Ethanol oxidised into ethanoic acid equation

a) CH3CH2OH + [o] ---> CH3CHO + H2O

b) CH3CH2OH + 2[o] ---> CH3COOH + H2O

Ketone functional group

Aldehyde functional group

Propan-2-ol oxidised to a ketone

CH3CHOHCH3 + [o] ---> CH3COCH3 + H2O

What determines whether a molecule can be oxidised or not

Carbon to which the oxygen is bonded to must have a removable hydrogen, eg propanone cannot be oxidised further

Test to distinguish propanoic acid from propan-1-ol

Add sodium hydrogen carbonate to both

Observation with propanoic acid: effervescence

Observation with propane-1-ol: no visible change

Show how combining the equations from these two methods can lead to the 1:2 mol ratio of carbon monoxide to hydrogen required for this synthesis of methanol (2)

Method 1 CH4 + H2O → 2CO + 3H2

Method 2 CH4 + CO2 → 2CO + 2H2

A method which shows (see below) OR states in words that two times the first equation + the second equation gives the correct ratio

Vegetable oils, which contain unsaturated compounds, are used to make margarine. Identify a catalyst and a reagent for converting a vegetable oil into margarine

Catalyst: Nickel/platinum/palladium

Reagent: Hydrogen/h2

State what is meant by the term hydration (1)

Addition of water/steam

Give a suitable reagent and reaction conditions for the oxidation of ethanol to form the carboxylic acid as the major product

Potassium dichromate

H2SO4

Most of the ethene used by industry is produced when ethane is heated to 900°C in the absence of air. Write an equation for this reaction

C2H6 →C2H4 +H2

Test for primary, secondary and tertiary alcohols

Primary - potassium dichromate turns it from orange to green

Secondary - potassium dichromate turns from orange to green

Tertiary - potassium dichromate remains orange

Test and observation to distinguish between primary and secondary alcohols

Primary are aldehydes/carboxylic acid

Secondary are ketones

Test for aldehyde

Tollens reagent ---> silver mirror

Feelings solution ---> red precipitate

In a case of a positive test an aldehyde, hence a primary alcohol is present.

Ketones (secondary) would not respond to these tests

Test to distinguish between silver nitrate and aqueous sodium nitrate

Reagent: any soluble chloride including HCl

Observation with silver nitrate: white precipitate

Observation with sodium nitrate: no visible change

Can use sodium iodide/bromide

Some alcohols can be oxidised by an acidified solution of potassium dichromate(Vl). Aldehydes can be oxidised by Tollens' reagent or by Fehling's solution.

An unknown pure liquid A contains only a single alcohol.Outline a simple procedure to allow you to determine whether A is a primary, a secondary or a tertiary alcohol

-Mix the alcohol with warm potassium dichromate to test for the presence of tertiary alcohol, there will be no visible change

-Distillation of initial product needed for test of primary/secondary

-Tollens reagent will result in silver mirror of aldehyde (primary alcohol present)

Write an equation for the oxidation of butane-1,4-diol into butanedioic acid

HOCH2CH2CH2CH2OH + 4[O] HOOCCH2CH2COOH + 2H2O

The energy stored in fuels can be compared using energy density values measure in kJdm-3

Calculate the energy density of butan-1-ol

Enthalpy of combustion of butan-1-ol = -2676 kJmol-1

Density of butan-1-ol = 0.810kg moldm-3

Mr of butan-1-ol

Amount of butan-1-ol in 1dm3 = 810/74 = 10.95

10.85 x -2676 = 29300

The students then set up the apparatus shown in the diagram below and placed the aqueous mixture in the round bottomed flask.

Describe how the students would use this apparatus to collect a sample of ethanol. Include in your answer the functions of the parts of the apparatus labelled A, B and C.

Stage 1 - turn on the water, heat the flask with a bunsen burner. This causes water and ethanol vapours to be produced

Stage 2 - Vapours pass up the fractionating column A. Water and ethanol are separated in column A

Water condenses back into the flask in column A

Stage 3 - observe the thermometer at B to keep the temperature at or below the boiling point of ethanol

Only ethanol vapour passes into the condenser.

Use the condenser at part C to cool vapours and condense the ethanol back int liquid