Alcohols

Question 1

why are alcohols less volatile and have higher melting point than alkanes

Answer 1

• polar -OH on the alcohol can form much stronger H bonds with other -OH groups, on top of the weak London forces present in alkanes
• more energy needed to overcome these bonds

Question 2

why are alcohols more soluble in water than alkanes

Answer 2

• the polar -OH group of alcohol molecules, not present in alkanes, can H bond with water

Question 3

what makes the -OH group polar

Answer 3

• electronegativity of O allows it to attract bonding electrons more than H, giving the H a slightly +ve charge

Question 4

why does solubility of alcohols decrease with increase in size

Answer 4

• in larger alcohols, most of the molecule is a non-polar C chain so there’s less attraction for the polar H2O molecules

Question 5

what kind of combustion do alcohols undergo and what does it form

Answer 5

• complete combustion
• exothermic reaction
• produces water and carbon dioxide

Question 6

what reagents is used to oxidise alcohols

Answer 6

• K2Cr2O7/ H2SO4
• Cr2O7^2-/ H+

Question 7

Which type of alcohols can’t be oxidised and what would be the observation

Answer 7

3*
- solution remains orange

Question 8

reaction conditions, general equation and observation of oxidation of 1* alcohol to aldehyde

Answer 8

• dilute acid, gentle heating, distillation
• 1* alcohol + [O] –(CrO7^2-/ H+)–> aldehyde + water
• colour change from orange to green

Question 9

reaction conditions, general equation and observation of oxidation of 1* alcohol to carboxylic acid

Answer 9

• conc acid, vigorous heating, reflux
• 1* alcohol + 2[O] –(CrO7^2-/ H+)–> carboxylic acid + water
• colour change from orange to green
• ONLY OXIDATION THAT NEEDS 2[O]

Question 10

reaction conditions, general equation and observation of oxidation of 2* alcohol to ketone

Answer 10

• conc acid, vigorous heating, reflux
• 2* alcohol + [O] –(CrO7^2-/ H+)–> ketone + water
• colour change from orange to green

Question 11

reaction conditions, general equation and observation of oxidation of aldehydes to carboxylic acids

Answer 11

• dilute acid, vigorous heating, reflux
• aldehyde + [O] –(CrO7^2-/ H+)–> carboxylic acid

Question 12

what are elimination reactions

Answer 12

when molecule loses atoms, forming a C=C

Question 13

general equation for the elimination of alcohols to form alkenes

Answer 13

alcohol –(conc H2SO4 and heat under reflux)–> alkene + H2O
- there are often 2 possible alkene products from one elimination reaction depending on which side of the -OH group the H is eliminated from
- E/Z isomerism may arise from this

Question 14

elimination of alcohols to form alkenes is also an example of which type of reaction

Answer 14

dehydration

Question 15

general formula for alcohols

Answer 15

CnH(2n+2)O