ALCOHOLS

At the end of this chapter, students should be able to:

Draw the structures, classify and name the hydroxy compounds according to IUPAC nomenclature

Explain physical properties of alcohol.Explain the reaction of alcohol:

1. Formation of halogenoalkanes2. Reactions with sodium3. Oxidation4. Dehydration5. Esterification

• Explain the identification tests to distinguish classes of alcohols using:1. Lucas reagent2. KMnO4/H+ or Cr2O7

2-/H+

INTRODUCTIONHydroxy compounds include aliphatic alcohols,

phenols and aromatic alcohols.Aliphatic alcohols are organic compounds with

at least one hydroxy (-OH) group attached to the alkyl group.

Functional group – hydroxy (-OH) group.General formula: CnH2n+1OH

IUPAC NomenclatureFind the longest carbon chain containing

the carbon with the -OH group.Drop the -e from the alkane name, add -ol.Number the chain, starting from the end

closest to the -OH group.Number and name all substituents. =>

Name these:

CH3 CH

CH3

CH2OH

CH3 C

CH3

CH3

OH

CH3 CH

OH

CH2CH32-methyl-1-propanol

2-methyl-2-propanol

2-butanol

OH

Br CH3

3-bromo-3-methylcyclohexanol =>

Classification

Primary: carbon with –OH is bonded to one other carbon.Secondary: carbon with –OH is bonded to two other

carbons.Tertiary: carbon with –OH is bonded to three other

carbons.Aromatic (phenol): -OH is bonded to a benzene ring.

H CH3 CH3

CH3-C-OH CH3-C-OH CH3-C-OH H H CH3 Ethanol 2-propanol 2-methyl-2-

propanolPrimary (1o) secondary (2o) tertiary (3o)alcohol alcohol alcohol

Physical PropertiesAlcohols are polar compounds

they interact with themselves and with other polar compounds by dipole-dipole interactions

Dipole-dipole interaction:Dipole-dipole interaction: the attraction between the positive end of one dipole and the negative end of another

O

HH

H

CH

+-

+

Physical PropertiesHydrogen bondingHydrogen bonding: when the positive end

of one dipole is an H bonded to F, O, or N (atoms of high electronegativity) and the other end is F, O, or Nthe strength of hydrogen bonding in water is

approximately 21 kJ (5 kcal)/molhydrogen bonds are considerably weaker

than covalent bondsnonetheless, they can have a significant

effect on physical properties

Hydrogen Bonding

Physical PropertiesIn relation to alkanes of comparable size

and molecular weight, alcoholshave higher boiling pointsare more soluble in water

The presence of additional -OH groups in a molecule further increases solubility in water and boiling point

Solubility of alcohols in water decreases with increasing carbon number

Formation of halogenoalkanesReactions with sodium

OxidationDehydration

Esterification

REACTIONS

Reagent: Hydrogen halide (HX) or phosphorus halide (PX3 or PX5) or SOCl2

1.With hydrogen halide- the alcohol is refluxed with NaX and concentrated H2SO4 (to produce HX)

- eg: NaCl + H2SO4 HCl + NaHSO4

C2H5OH + HCl C2H5Cl + H2O

- most suitable for chlorides and bromides-

Formation of Halogenoalkanes

2. With phosphorus halide & sulphur dichloride oxide.- iodoalkanes can be made by warming a alcohol with a mixture of red phosphorus and iodine.- phosphorus (III) iodide is first formed which then reacts with alcohol

2P + 3I2 2PI3

3CH3OH + PI3 3CH3I + P(OH)3

- chloroalkanes can be made by reacting alcohols with phosphorus (III) chloride, PCl3, phosphorus (v) chloride, PCl5, or sulphur dichloride oxide, SOCl2.

CH3CH2OH +PCl5 CH3CH2Cl + HCl + POCl3CH3CH2OH + SOCl2 CH3CH2Cl + HCl + SO2

Reaction with sodiumAlcohols react with sodium to give hydrogen

gasEg: C2H5OH + Na C2H5O-Na+ + 1/2H2

sodium ethoxide

-observation: sodium sinks and bubbles of H2(g) evolved.

OxidationReagent : acidified K2Cr2O7 ( or acidified KMnO4)Condition: heat or refluxObservation: orange aq. K2Cr2O7 turns green ( due to

formation of Cr3+ ions) or purple aq. KMnO4 decolourised (due to formation of Mn2+

ions).Primary alcohols

- are oxidised to aldehydes an then to carboxylic acidsCH3CH2OH + [O] CH3CHO(ethanal) + H2O

Cr2O72-/H+

reflux CH3COOH

(ethanoic acid)

Secondary alcohols- Are oxidised to ketonesEg: H Cr2O7

2-/H+

CH3-C-CH3 + [O] CH3C=O + H2O

OH reflux CH3

propanoneTertiary alcohols - are not oxidised- Bcoz tertiary alcohols do no have any hydrogen

atom to be removed from the carbon atom to which the –OH group is attached

Thus, oxidation is used to distinguish primary, secondary, and tertiary alcohols.

DehydrationReagent: excess concentrated H2SO4

Condition: heat at about 170oCProduct: alkenes

excess c. H2SO4

Eg: CH3CH2OH CH2=CH2 + H2O

ethanol heat at 170oC etheneIf excess ethanol is used & the temperature is

kept about 140oC, ether is produced instead of ethene

c. H2SO4

2CH3CH2OH CH3CH2OCH2CH3 + H2Oethanol (excess) heat at 140oC ethoxyethane (ether)

EsterificationReagent: carboxylic acid or acyl halideProduct: sweet-smelling ester1.With carboxylic acid- condition: reflux in the presence of a small amount of

concentrated H2SO4

Eg: CH3CH2O + CH3C=O CH3C=O + H2O

H OH OCH2CH3

ethanol ethanoic acid ethyl ethanoate

- Concentrated H2SO4 is added as a catalyst

it also helps to increase the equilibrium yield of ester by removing the H2O formed and so, shifts the equilibrium to the right.

2. With acyl halide- condition: room temperatureeg:

CH3CH2O + CH3C=O CH3C=O + HCl

H Cl OCH2CH3

ethanol ethanoyl chloride ethyl ethanoate

- this reaction is very much faster than that with carboxylic acid because ethanoyl chloride is very reactive.

Identification tests to distinguish classes of alcohols1. Lucas reagent

no visible reaction: primarysolution turns cloudy in 3-5 minutes:

secondarysolution turns cloudy immediately

2. KMnO4/H+ or Cr2O72-/H+ (discussed in oxidation

reaction)