14 Alcohols

features of a homologous series

• each successive member differs by a CH2 group

• similar chemical properties and gradually changing physical properties

• same general formula and functional group

functional grp of alcohols

-OH

how can ethanol be formed from ethene

when reacted with steam H₂O_(g), 300C, acid catalyst H₃PO_4, 60-70atm

what priority does -OH have in naming

• highest priority so far - needs to have the smallest number

  • alkenes C=C

  • halogens

  • alkyl = lowest

what is methanol used for

• a high performance fuel due to efficient combustion

• starting material in many industrial syntheses

• can be converted into polymers, paints, solvents, adhesives and other useful products

what is ethanol used for

• alcoholic drinks

• a fuel, solvent or feedstock (raw material)

where are the polar bonds on alcohols

on the OH group

Oδ-

Hδ+

miscible def

does a liquid mix with water

are alcohols miscible w water

• yes

• like dissolves like

• alcohols are polar

diagram of hydrogen bonding between ethanol molecule and water

how does solubility change as hydrocarbon chain increases

• the larger the chain the more LF

• greater IMF

• harder to pull molecules apart to dissolve

• solubility decreases as hydrocarbon chain gets longer

influence of the OH group decreases as the chain length increases, not as energetically favourable

why does methanol have a higher bpt than methane

• methanol has hydrogen bonding between molecules

• methanol also has permanent dipole dipole interactions because its polar

• more energy required to overcome IMF

• methane doesn’t have HB or pddi between molecules

compare bpt of methanol to methane

methanol has a higher bpt than methane

compare volatility of methanol to methane

methane has a lower volatility

(same reason as bpt)

compare solubility of methanol to methane

• methanol more soluble

• has an OH group that can form HB w water

• methanol is a polar molecule

• like dissolves like

what effect does increasing carbon chain length have on alkanes vs alcohols

• LF can accumulate and be stronger than HB

• HB are individually stronger than LF

how are alcohols classified

based on the no. of carbons the C bonded to the OH group is bonded to

a primary alcohol

• the -OH is joined to a carbon that isn’t joined to more than 1 carbon

a secondary alcohol

• the -OH is joined to a carbon that is joined to 2 other carbons

a tertiary alcohol

• the -OH is joined to a carbon that is joined to 3 other carbons

reactions of alcohols

• combustion

• oxidation

• dehydration

• substitution

what functional groups can be produced in oxidation of alcohols

• aldehydes

• ketones

• carboxylic acids

what is the ending for an aldehyde

-al

what is the ending for a ketone

-one

functional group of an aldehyde

• on the end carbon

functional group of a ketone

• NOT on an end carbon

structural formula for an aldehyde

-CHO

structural formula for a ketone

-CO-

IMF between aldehydes

• LF

• permanent dipole-dipole interactions (C=O)

IMF between ketones

• LF

• permanent dipole-dipole interactions

IMF between carboxylic acids

• LF

• permanent dipole-dipole interactions

• HB

bpts of alkanes, aldehydes and alcohols

• alkane (lowest) (only LF)

• aldehyde (permanent dipole-dipole interactions)

• alcohol (highest) (HB)

why do solubility of aldehydes and ketones decrease as the molecules get bigger

• more HBs in water need to be broken to fit the aldehyde/ketone between the water molecules

• however only 1 HB gets formed in exchange

• not energetically favourable

conditions for oxidation of alcohols

• acidified (by adding dilute H₂SO₄) potassium (or sodium) dichromate (VI)

• heating of mixture

what colour does potassium dichromate (VI) go if it is reduced

orange to green

how is the oxidising agent written in equations

[O]

what do the products of oxidation of alcohols depend on

whether it was a primary, secondary or tertiary alcohol

possible products of oxidation of a primary alcohol

• aldehyde

  • gentle heating

  • distilled off as formed to collect

• carboxylic acid

  • excess oxidising agent

  • heating under reflux

  • conc. sulfuric acid

stuff needed for formation of an aldehyde from a primary alcohol

• gentle heating

• K₂Cr₂O₇/H₂SO₄ acidified potassium dichromate (VI)

• distill

stuff needed for formation of a carboxylic acid from a primary alcohol

• excess oxidising agent 2[O]

• (K₂Cr₂O₇/H₂SO₄ acidified potassium dichromate (VI))

• heating under reflux

• concentrated H₂SO₄

stage 1 of oxidation of a primary alcohol

• gently heating with acidified potassium dichromate (VI)

• produces aldehyde

• aldehyde separated from mixture as soon as it forms to prevent stage 2 taking place

• can be separated by distillation

stage 2 of oxidation of a primary alcohol

• forms a carboxylic acid

• requires vigorous oxidation w excess oxidising agent

• heating under reflux

• concentrated sulfuric acid

oxidation stage 1 w ethanol equation

oxidation stage 2 w ethanol equation

overall equation for oxidation of a primary alcohol

how does reflux ensure the carboxylic acid is formed rather than an aldehyde

the aldehyde can’t escape as it’s repeatedly condensed back

what is the colour change of oxidation of a primary alc and why

orange to green

• oxidising agent

• Cr reduced

what is formed when a secondary alc is oxidised

ketones

what are secondary alcohols oxidised with

acidified potassium dichromate (VI) K₂Cr₂O₇/H₂SO₄

• the H₂SO₄ is dilute

equation for oxidation of secondary alc

why don’t tertiary alcs oxidise

• don’t oxidise

• there’s no H on the adjacent C that can be lost

what is meant by a dehydration reaction

alcohol → alkene

• loss of H2O molecule

• elimination reaction

what is meant by heating under reflux

constant reaction between reactants

• evaporating then condensing

• completing without losing volatile reactant

what r the benefits of heating under reflux

good for volatile substances (liquids → gases)

dehydrating agents

• conc. H₂SO₄

• conc. H₃PO₄

• heated Al₂O₃

conditions for dehydration of alcohols

• a dehydrating agent (H₂SO_4, H₃PO_4, Al₂O₃₎

• alc heated under reflux

what kind of reaction is dehydration of alcs

elimination

what happens in a dehydration of an alc rxn

• C=C is reformed

• alkene + water produced

how would you prove the product is an alkene

• add orange bromine water

• goes from orange to colourless if alkene

how many possible products can be made in the dehydration of butan-2-ol

2 depending on which H next to the OH group is taken

• but-1-ene

• but-2-ene

what is produced when alcohols react with hydrogen halides

haloalkanes

what happens in a substitution reaction with HBr

• alc is heated under reflux with dilute H₂SO₄ and a sodium halide

• the HBr is formed in situ

stuff needed for substitution w hydrogen halides + alcohol

• a sodium halide e.g. NaBr(s)

• dilute H₂SO₄

• heated under reflux

how is the HX formed in an alcohol + hydrogen halide rxn

in situ (in place)

step 1 of substituion

step 2 of substitution