Alcohols

What is the general formula of an alcohol?

CnH2n+1OH

What prefix is used is used if there are other functional groups in a molecule?

Hydroxy-

What is the bond angle of C-O-H and why?

104.5 degrees because the oxygen atom has 2 bonding pairs of electrons and 2 lone pairs, which cause increased repulsion compared to electron pairs in a covalent bond.

How are alcohols classified as primary, secondary or tertiary?

By how many other R groups there are, primary has one R group attached to the C with the hydroxyl groups, secondary has 2 and tertiary has 3

Why do alcohols have higher melting points than alkanes of similar molecular mass?

The hydroxyl group in alcohols means that hydrogen bonding occurs between molecules which requires more heat energy to break. This is because oxygen is more electronegative than hydrogen, so there is an uneven distribution of charge resulting in a delta + H and delta - O.

What is the trend of solubility in alcohols and why?

Alcohols contain both polar and non-polar regions. The polar region is from the OH group being able to hydrogen bond to water molecules, but the non-polar region of the carbon chain cannot. Therefore alcohols with short carbon chains are soluble in water as the hydrogen bonding predominates but in longer chain molecules the alcohol becomes insoluble in water as the hydrocarbon chain predominates.

What is ethanol used for?

Used as an intermediate in the manufacture of other organic chemicals, solvent in cosmetics like perfumes, used to manufacture drugs and detergents.

How is ethanol made industrially and what is the name for this?

Reacting ethene (made from cracking crude oil) with steam and a catalyst of concentrated phosphoric acid. Called hydration of ethene

How is ethanol made in the drinks industry?

Made from glucose by fermentation.

Draw the mechanism for hydration of ethene using steam:

Outline the process of ethanol production by fermentation and the equation:

• Sugar is broken down via anaerobic respiration

C6H12O6 → 2C2H5OH + 2CO2

What reaction conditions are used for fermentation of glucose and why?

• Temperature of 35 degrees is used, because reaction is too slow at low temperatures, but if temperature is above optimum then the enzymes become denatured

• Anaerobic conditions are used because if O2 is present no ethanol is produced and air is kept out of the reaction vessel to prevent oxidation of ethanol to ethanoic acid

• Presence of yeast

Compare the processes of hydration of ethene and fermentation of glucose:

The starting material for the hydration is crude oil which is non renewable, but sugars are renewable, but rate of hydration is faster than fermentation, and product purity is lower in fermentation, as well as the atom economy.

Biofuel definition:

Fuel derived from, or produced from renewable biological sources

Carbon neutral fuel definition:

A fuel in which the CO2 released when it is burnt is balanced by the CO2 absorbed by the plant from which it was obtained during photosynthesis

Why might ethanol not be a carbon neutral fuel?

• CO2 may be released in transporting of fuels, and it requires energy to maintain reaction conditions at 35 degrees

What are elimination reactions?

One in which a small molecule leaves the parent molecule

What are the conditions for the dehydration of alcohols?

Can be dehydrated with excess hot concentrated H2SO4 or phosphoric acid, or by passing their vapours over heated aluminium oxide

What does the dehydration of alcohols produce?

Alkene

What is the mechanism for the dehydration of propanol?

What is the method for the production of cyclohexene by dehydration and distillation of cyclohexanol?

1. Add the cyclohexanol into a round bottomed flask that has been weighed, then reweigh with the cyclohexanol. Add concentrated phosphoric acid and anti-bumping granules.

2. The flask should have a still head containing a thermometer, and attach this to a condenser with an ice-cooled collecting vessel to reduce product evaporation.

3. Heat the flask gently and collect sample at boiling point of desired produce.

4. Pour distillate into separating funnel and add sodium chloride, shake mixture and allow layers to separate.

5. Run off lower layer into beaker then transfer upper layer into conical flask.

6. Add anhydrous calcium chloride to remove water then transfer to a dry container with known mass, weigh it to determine dry product mass.

What is the purpose of anti-bumping granules?

Prevent large bubbles from forming and ensure that the liquid doesn’t boil too vigorously

What products are formed from oxidising primary alcohols?

Aldehydes when gently oxidised, then further oxidised to carboxylic acids.

What are secondary alcohols oxidised to?

Ketones, which are not oxidised further.

What product is formed from a primary alcohol when the alcohol is in excess and is distilled?

Aldehyde

Why are tertiary alcohols resistant to oxidation?

There is no hydrogen attached on the carbon with the hydroxyl group.

What reagent is used to oxidise alcohols to aldehydes and ketones?

A solution of potassium dichromate and dilute sulfuric acid.

What is the oxidising agent used to oxidise alcohols, and what are they reduced to?

The orange dichromate (VI) ions, and they are reduced to green chromium (III) ions

What is the half equation for the reduction of dichromate ions?

Cr2O7²⁻ + 6e- + 14H+ → 2Cr³⁺ + 7H2O

What is the procedure for oxidising a primary alcohol to an aldehyde?

Dilute acid and potassium dichromate as a limiting reactant is used. The mixture is gently heated using distillation apparatus, but the receiver is cooled in ice to reduce evaporation of the product.

Give an equation for the reaction between ethanol and an oxidising agent (O)?

CH3CH2OH + [O] → CH3CHO + H2O

What conditions are used to fully oxidise a primary alcohol to a carboxylic acid?

• Concentrated sulfuric acid and potassium dichromate in excess. The mixture is refluxed.

What does reflux mean?

An experimental method where vapour that condenses drips back into the reaction flask.

How does reflux work when oxidising a primary alcohol?

Any ethanol or ethanal vapour will condense and drips back into the flask until eventually all of it is oxidised to carboxylic acid. The carboxylic acid can then be condensed off the mixture using distillation.

Give the equation for the reaction of ethanol to form a carboxylic acid using an oxidising agent [O]:

CH3CH2OH + 2[O] → CH3COOH + H2O

What functional group do both ketones and carboxylic acids share?

Carbonyl group (C=O), in aldehydes it is at the end of the chain but in ketones it is in the middle.

What is the difference between aldehydes and ketones in terms of oxidation?

Aldehydes can be oxidised to carboxylic acids, ketones are not changed by gentle oxidation.

What is Tollen’s reagent?

Gentle oxidising agent, and is a solution of silver nitrate in aqueous ammonia. It oxidises aldehydes and not ketones

Describe the procedure of the silver mirror test:

When the sample is warmed with Tollen’s reagent, if an aldehyde is present a deposit of metallic silver is formed on the inside of the test tube because Ag+ ions are reduced to metallic silver.

Describe the procedure of the Fehling’s test:

It contains blue copper complex ions that oxidise aldehydes and not ketones. If aldehyde is present the solution changes from blue to brick red (precipitate of copper oxide).