Chemistry Topic 14 alcohols

14.1- 2

What is an alcohol?

A homologous series of organic compound containing the OH hydroxyl group with the general formula of CnH2n+1OH.

What are the two ways of obtaining alcohol?

1. Fermentation from sugars by yeast

2. Hydration of alkenes using steam

Compare the polarity of alcohol and alkane.

Alcohol is polar because of the difference in electronegativity between oxygen and hydrogen.

While alkane is non polar because the electronegativity is similar so they have an equal sharing of electrons.

Compare the boiling points and volatility between an alochol and alkane with the same number of carbon atoms. (4)

Alcohols have higher boiling points because they have both hydrogen bonds and London forces so it requires more energy to break the bonds.

While alkanes only have London forces so less energy is needed to break the intermolecule forces.

The higher the boiling point, the lower the volatility. Volatility is a measure of how easy a substance canw be evaporated.

The higher the boiling point, the harder it will be evaporated so alcohol is less volatile than alkanes.

Compare the solubility of alkanes and alcohols in terms of their polarity and bonds.

Alkanes are insoluble because they only have weak London forces and they are non polar so they cannot overcome the strong forces between water molecules or form hydrogen bonds between water molecules.

Alcohol are soluble because they have polar OH groups and both strong hydrogen bonds and London forces that provides enough energy to form hydrogen bonds with water molecules.

Compare the solubility of larger alcohol chains and shorter alcohol chains.

Larger alcohol chains are less soluble than shorter alcohol chains because the proportion of non- polar molecules becomes larger, weakening the impact of the hydrogen bond.

Methanol and methane have a difference in boiling point around 200°C while the difference of boiling points in pentanol and pentane is only 50°C.

Why is the boiling point between pentanol and pentane smaller that the difference between methanol and methane?

Methanol and methane are smaller chains of alcohol and alkane compare to pentanol and pentane.

In alcohols, smaller hydrocarbon chains mean that the impact of hydrogen bond will be bigger.

The bigger the proportion of hydrogen bond there are in a molecule, the higher the boiling point.

There will be a bigger difference in boiling point between smaller alcohol hydrocarbon chains and the same length of alkane.

The difference of boiling points between alcohol and alkanes becomes smaller as carbon chain length increases.

Define primary, secondary and tertiary alcohol.

Alcohols in which the carbon atom with OH group is attached to 1, 2 or 3 alkyl groups.

What is an oxidising agents used when oxidising alcohols.

K₂Cr₂O₇ - potassium dichromate acidified with dilute sulfuric acid H₂SO₄

What happens to the ion in the oxidising agent when an alcohol is oxidised?

The orange Cr₂O₇^2- with Cr having an oxidation state of 6+ reduced to green Cr 3+ ions.

Why does the potassium dichromate needed to be acidified by dilute sulfuric acid? And why not use other acids?

For Cr to be reduced as the half equation needs H⁺ ions to form water as by- products.

Cr₂O₇^2- + 14H⁺ + 6e^- → 2Cr³⁺ + 7H₂O

Sulfuric acid is stable and does not get oxidised by dichromate whereas chloride ions in hydrochloric acid can be oxidised to form toxic chlorine gas.

What are 2 products (include their suffix and functional groups) primary alcohols could make during oxidation?

Aldehydes- suffix: -al, functional group= R-CHO

Carboxylic acid- suffix: -oic acid, functional group= R-COOH

Explain the functions of different distillation apparatus used in partial oxidation.

1. Pear- shaped flask- improves heat efficiency and allow every drop to be heated

2. Anti- bumping granule - ensures that boiling is not too violent

3. Dropping funnel and tap- control the slow addition of oxidising agent and ensuring that it is not triggering a futher step to make carboxylic acid

4. Thermometer- moniter temperature of the vapour and making sure it is only being partially oxidised into aldehydes

5. Condensor with the cool water in at the bottom and cool water out at the top to make sure it is always filled with cool water and to condense the vapour into liquid.

6. Ice water bath- keep the liquid product cool and avoid it evaporating back to vapour

Describe the process for the partial oxidation of making aldehydes.

Partial oxidation is carried under distilation as aldehyde wants to be seperated from the reaction mixture as it forms.

1. A dropping funnel with a tap is used to slowly release solutions of alcohol and potassium dichromate into a pear shaped flask with dilute sulfuric acid and anti bumping granules

2. A heating mantle is used to heat up the solutions in the pear shaped flask while a thermometer is placed on top of the flask where vapour usually passes through to moniter and ensuring that it is only partially oxidising not fully oxidising.

3. The aldehyde evaporates up the thermometer and enters the Liebig condensor

4. Aldehyde liquid flows into the conical flask soaking into an ice water bath

Describe the process for full oxidation of making carboxylic acid.

This process is carried by reflux as it allow the vapour to condense back to the mixture.

1. The reaction mixture containing dilute sulfuric acid, excess of potassium dichromate and a primary alcohol is placed in a pear shaped flask and heated with a heating mantle

2. The alcohol turns into aldehyde at first and the vapours rise to the top until it cools back in liquid and drops back to the flask

3. Aldehyde then reacts further with oxidising agent to form carboxylic acid.

What are the equations for making aldehydes, carboxylic acid and Ketones from ethanol, propan-1-ol and propan-2-ol?

1. ethanol = CH3CH2OH (OH group) + [O] → CH3CHO (R- CHO) +H2O

Lost 2 hydrogens and 2 electrons

2. propan-1-ol= CH3CH2CH2OH (OH group) + 2[O] → CH3CH2COOH (R-COOH) +H2O

One of the oxygen from the oxidising agent pulls off 2 hydrogen from the alcohol and form an aldehyde with a water molecule.

While the other oxygen from the oxidising agent joins to the C to H bond in the aldehyde to make a carboxylic acid (this step does not make any water molecules)

3. propan-2-ol = CH₃CHOHCH₃ + [O] → CH₃COCH₃ (R-C=O-R) + H₂O

The oxygen from the oxidising agent pulls off the 2 hydrogens from the second carbon with the OH group to make H2O and left with the second carbon attached to an oxygen molecule in a double bond.

Explain why tertiary alcohols do not undergo oxidation reactions?

The carbon attached to the OH group lack a hydrogen that can form a carbon to oxygen double bond because there are 3 alkyl groups and 1 OH group.

Define an elimination reaction.

A reaction where a small molecule left the parent molecule as a byproduct

How do elimination reaction happens in an alcohol and what condition is needed for this to happen?

Elimination reaction in a alcohol forms an alkene and it is also called a dehydration reaction, with water molecules being the byproduct.

This is heated out under a reflux with the use of an acid catalyst like conc sulfuric acid and phosphoric acid or aluminium oxide catalyst.

Describe the mechanism for the elimination reaction of alcohol to form an alkene.

1. Lone pair of electrons from the oxygen in the OH group is attracted to the H⁺ ion from the acid catalyst. (an arrow going from lone pair on oxygen to H⁺ ion)

2. The lone pair from oxygen now forms a bond with the H+ ion while an intermediate molecule of a positive oxygen atom is formed.

3. The pair of electrons in the covalent bond between the carbon and oxygen atom moves to the positive intermediate of the oxygen. (arrow going from the C to O bond to the positive intermediate)

4. Pair of electrons in the carbon to hydrogen bond move to the C to C bond, forming a double bond as well as releasing the hydrogen as H⁺ ( arrow from C to H bond to C to C bond)

5. At last, an alkene, a water molecule and a H⁺ ion are formed and the H⁺ ion can be regenerated.

Define a substitution reaction.

It is where an atom or a group of atoms in a molecule is replaced by a different atom or group

How do substitution reaction happens in an alcohol and what condition is needed for this to happen?

Alcohol react with hydrogen halides to form haloalkanes where a halide group (X) replaces the OH group.

The alcohol is heated under reflux with sulfuric acid and NaBr/ KBr

Describe the steps and mechanism for the substitution reaction of alcohol to form a haloalkane.

• NaBr and H₂SO₄ react together to form HBr and NaHSO₄

• HBr then react with the alcohol to form haloalkane

1. Lone pairs in the oxygen in OH group is attracted by the partial positive hydrogen of the HBr. (arrows going from lone pair to partial positive sign on hydrogen)

2. The pair of electrons from the H to Br bond goes to the Br and release it as Br^- with a lone pair

3. A bond forms between the O and H while a positive intermediate forms on the oxygen.

4. The lone pair of the Br^- goes to the carbon that is linked to the H2O intermediate.

5. The pair of electrons on the C to O bond then moves to the intermediate on the oxygen.

6. At last, the product of a haloalkane and a water molecule is formed.

Define esterification reaction.

This is a condensation reaction where a carboxylic acid and an alcohol react under reflux and anacid catalyst to form ester and water.

Describe the steps how an ester forms by removing water as byproduct.

The OH from the COOH group in the carboxylic acid reacts with the hydrogen in the OH group from the alcohol to form water.

A C (from carboxylic acid ) to O (from the alcohol) bond forms.

How do we identify esters?

They have a strong sweet or fruity smell

How do we name esters using names from the alcohol and carboxylic acid?

1. The alcohol name goes to the front (prefix) and change the -ol to -yl

2. Carboxylic acid name goes to the back (suffix), chaning -oic acid to oate.