FC21: Alcohols

Question 1

What is the functional group of an alcohol

Answer 1

hydroxyl functional group (-OH)

Question 2

What is the relative boiling/melting point of alcohols and why

Answer 2

relatively high for their size due to hydrogen bonding between alcohol molecules

Question 3

What is the relative solubility of alcohols and why

Answer 3

Alcohols are highly soluble in water as they can form strong interactions (H-bonding) between the molecules. They are miscible

Question 4

What is the oxidising agent used when oxidising alcohols

Answer 4

hot, aqueous, acidified potassium dichromate
K2Cr2O7 (aq) / H+(aq)

Question 5

What is the active part of potassium dichromate when oxidising alcohols

Answer 5

Cr2O7 2-

Question 6

What do primary alcohols form when oxidised, and what do they form if they are left with the oxidising agent for longer

Answer 6

primary alcohols oxidise to form aldehydes, which further oxidise to form carboxylic acids

Question 7

What is the colour change of the oxidising agent when oxidising alcohols

Answer 7

potassium dichromate changes colour from orange to green

Question 8

What do we use to acidify potassium dichromate for the oxidation of alcohols and why

Answer 8

sulfuric acid, not hydrochloric acid because chloride ion in HCl would oxidise to toxic chlorine gas

Question 9

At what position of the chain in an aldehyde is the doubly bonded oxygen?

Answer 9

at start or end of chain as it is only formed by primary alcohols therefore no position number

Question 10

At what position of the chain in a carboxylic acid is the COOH functional group?

Answer 10

at start or end of chain as it is only formed by primary alcohols therefore no position number

Question 11

What do secondary alcohols form when oxidised, and what do they form if they are left with the oxidising agent for longer

Answer 11

secondary alcohols oxidise to form ketones, and do not further oxidise when left with the oxidising agent

Question 12

What do tertiary alcohols form when oxidised, and what do they form if they are left with the oxidising agent for longer

Answer 12

tertiary alcohols are not oxidised by hot, aqueous, acidified potassium dichromate

Question 13

At what position of the chain in a ketone is the doubly bonded oxygen?

Answer 13

anywhere but not the start or end of the chain as it is only formed by secondary alcohols

Question 14

How can you set up an apparatus to produce a higher yield of aldehyde

Answer 14

use distillation
set up a stand, boss and a clamp. clamp a pear shaped flask containing the primary alcohol, oxidising agent, and anti-bumping granules
put a thermometer connector with side arm into the pear shaped flask.
put a thermometer into the thermometer connector making sure the thermometer bulb is at the exit of the side arm
put a condenser around the side arm, cold water in one end and out the other end.
attach a delivery tube and a collecting vessel at the end of the tube.
heat the pear shaped flask

Question 15

How does using distillation produce a higher yield of aldehyde

Answer 15

any aldehyde formed will boil at a lower temperature than the original primary alcohol and other components in the mixture, as it does not form hydrogen bonding or ion dipole bonds, but weaker permanent dipole-dipole forces. Therefore the aldehyde would boil and condense into the collecting vessel

Question 16

How can you set up an apparatus to produce a higher yield of carboxylic acid

Answer 16

use refluxing
set up a stand, boss and clamp. clamp a pear shaped flask containing primary alcohol, oxidising agent and anti-bumping granules
Put a condenser just into the opening of the pear shaped flask, cold water going in one end and out the other end
Heat the pear shaped flask

Question 17

How does heating under reflux produce a higher yield of carboxylic acid

Answer 17

heating under reflux is continuous boiling and condensing of a reaction mixture using a condenser, so any unwanted aldehyde formed is condensed back into the oxidising agent to further oxidise into carboxylic acid

Question 18

What do anti-bumping granules do and why are they used

Answer 18

prevents super-heating of the reaction mixture by providing nucleation sites for bubbles to form
This encourages calmer boiling and prevents the mixture boiling over into the condenser

Question 19

why are alcohols polar molecules

Answer 19

there is a permanent dipole along the O-H bond due to the large difference in electronegativity values of O and H. The molecule is not completely symmetrical, so there is an overall dipole along the molecule and so alcohols are polar

Question 20

How can you make alcohols from alkenes?

Answer 20

hydration
alkene + water with concentrated phosphoric acid-> alcohol

Question 21

How can you make alcohols from haloalkanes? what is the mechanism for this reaction

Answer 21

alkane + sodium hydroxide with heat and sodium hydroxide (aq) -> alcohol + sodium halide
nucleophilic substitution

Question 22

describe how you could prepare an alcohol from an alkane in two steps

Answer 22

1. alkane + halogen under UV light -> haloalkane + hydrogen halide
2. haloalkane + sodium hydroxide with heat and sodium hydroxide (aq) -> alcohol + sodium halide

Question 23

What is formed when alcohols are heated with concentrated sulfuric acid and what type of reaction is this

Answer 23

alkene and water
dehydration / elimination

Question 24

How can you convert an alcohol into a haloalkane and what type of reaction is this

Answer 24

add sodium halide in the presence of concentrated sulfuric acid
substitution reaction