Alcoholsalcohols

AlcoholsAlcohols

Chapter 10

Structure of Alcohols

• The functional group of an alcohol is H an -OH group bonded to an sp3 O 108.9° hybridized carbon. C H – Bond angles about the hydroxyl oxygen H H atom are approximately 109.5°. • Oxygen is sp3 hybridized. –Two sp3 hybrid orbitals form sigma bonds to a carbon and a hydrogen. – The remaining two sp3 hybrid orbitals each contain an unshared pair of electrons.

2 Nomenclature of Alcohols

• IUPAC names – The parent chain is the longest chain that contains the OH group. – Number the parent chain to give the OH group the lowest possible number. – Change the suffix -etoe -ol.ol • Common names – Name the alkyl group bonded to oxygen followed by the word alcohol.alcohol

Nomenclature of Alcohols

OH o o 1o OH 1 2 OH 1-Propanol 2-Propan ol 1-Bu tanol (Pro py l alco ho l) (Isoprop yl alcoh ol) (Bu tyl alcoh ol)

OH 2o OH 1o OH 3o 2-Butanol 2-M eth yl-1-p ropan ol 2-M eth yl-2-p ropan ol (sec-Butyl alcohol) (Isobutyl alcohol) (tert -Butyl alcohol)

4 Nomenclature of Alcohols

• Compounds containing more than one OH group are named diols, triols, etc.

CH2 CH2 CH3 CHCH2 CH2 CHCH2 OH OH HO OH HO HO OH 1,2-Ethanediol 1,2-Propanediol 1,2,3-Propanetriol (Ethylene glycol) (Propylene glycol) (Glycerol, Glycerine)

Physical Properties

• Alcohols are polar compounds.

– They interact with themselves and with other polar compounds by dipole-dipole interactions. • Dipole-dipole interaction: The attraction between the positive end of one dipole and the negative end of another.

6 Physical Properties

• Hydrogen bonding: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 N. – The strength of hydrogen bonding in water is approximately 21 kJ (5 kcal)/mol. – Hydrogen bonds are considerably weaker than covalent bonds. – Nonetheless, they can have a significant effect on physical properties.

Hydrogen Bonding E.G. The association of ethanol molecules in the liquid state by hydrogen bonding.

8 Physical Properties

• Ethanol and dimethyl ether are constitutional isomers. • Their boiling points are dramatically different – Ethanol forms intermolecular hydrogen bonds, which are attractive forces between its molecules, resulting in a higher boiling point. – There is no comparable attractive force between molecules of dimethyl ether.

CH3 CH2 OH CH3 OCH3 Ethanol Dimethyl ether bp 78°C bp -24°C

Physical Properties

• In relation to alkanes of comparable size and molecular weight, alcohols: – have higher boiling points. – are more soluble in water. • The presence of additional -OH groups in a molecule further increases solubility in water (polar solvents) and boiling points.

10 Acidity of Alcohols

In dilute aqueous solution, alcohols are weakly acidic.

+ + : CH O:– + CH3 OH O H 3 H O H H H

- + [CH3 O ][H3 O ] - 15.5 Ka = = 10 [CH3 OH]

pKa = 1 5.5

Acidity of Alcohols

Structural Compound Formula pKa Hydrogen chloride HCl -7 Stronger acid Acetic acid CH3 COOH 4.8

Methanol CH3 OH 15.5

Water H2 O 15.7

Ethanol CH3 CH2 OH 15.9

2-Propanol (CH3 ) 2 CHOH 17 Weaker 2-Methyl-2-propanol (CH3 ) 3 COH 18 acid

Also given for comparison areK apvalues for water, acetic acid, and hydrogen chloride.

12 Reaction with Metals

• Alcohols react with Li, Na, K, and other active metals to liberate hydrogen gas and form metal alkoxides.

+ 2CH3 OH + 2Na 2CH3 O Na + H2 Sodium methoxide (MeO Na+)

Reaction with Metals

• Alcohols are also converted to metal alkoxides by reaction with bases stronger than the alkoxide ion. – One such base is sodium hydride.

+ + H CH3 CH2 OH+ Na H CH3 CH2 O Na + 2 Ethanol Sodium Sodium ethoxide hydride

2 ether; THF or alkanes (reaction is irreversible)

14 Reaction with HX

– 3° alcohols react very rapidly with HCl, HBr, and HI.

25°C OH + HCl Cl + H2 O

2-Methyl-2- 2-Chloro-2- propanol methylpropane

Reaction with HX – Low-molecular-weight 1° and 2° alcohols are unreactive under these conditions. – 1° and 2° alcohols require concentrated HBr and HI to form alkyl bromides and iodides. H O 2 Br OH + HBr+ H2 O reflux 1-Butanol 1-Bromobutane

OH X HX (conc)

R R(H) Heat R R(H) H H 1o &2o ROH 1o &2o Halides 16 Reaction with HX - SN1

Step 1: Proton transfer to the OH group gives an oxonium ion. rapid and CH CH3 H : 3 + reversib le CH -C-OH + HOH CH -C O + :O H 3 3 + CH3 H CH3 H H

Step 2: Loss of H2O gives a carbocation intermediate. slow, rate H CH H CH 3 determining 3 CH -C O CH -C+ + :O 3 + 3 SN 1 CH3 H CH3 H A 3° carbocation intermediate 17

Reaction with HX - SN1

Step 3: Reaction of the carbocation intermediate (an electrophile) with halide ion (a nucleophile) gives the product.

CH3 CH3 fast CH3 -C+ + :Cl CH3 -C-Cl CH3 CH3 2-Chloro-2-methylpropane (tert-Butyl chloride)

18 Reaction with HX – SN2

1°alcohols react with HX by an SN2 mechanism. Step 1: Rapid and reversible proton transfer.

rapid and H + reversible + + O H RCH2 -OH + HO H RCH2 -O H H H

Step 2: Displacement of HOH by halide ion.

H slow, rate H + determining - RCH -Br + : Br: + RCH2 -O 2 O H SN2 H 19

Reaction with PBr3 – SN2

• An alternative method for the synthesis of 1° and 2° bromoalkanes is reaction of an alcohol with phosphorous tribromide. – This method gives less rearrangement than with HBr.

20 Reaction with PBr3 – SN2

Step 1: Make a bond between a nucleophile and an electrophile and simultaneously beak a bond to give stable molecules or ions. Formation of a protonated dibromophosphite

converts H2O, a poor leaving group, to a good leaving group.

Step 2: Make a bond between a nucleophile and an electrophile and simultaneously beak a bond to give stable molecules or ions.

Reaction with SOCl2

• Thionyl chloride is the most widely used reagent for the conversion of 1° and 2° alcohols to alkyl chlorides. – A base, most commonly pyridine or triethylamine, is added to catalyze the reaction and to neutralize

the HCl. ( NEt3 or )

OH pyridine + SOCl2 1-Heptanol Thionyl chloride Cl + SO2 + HCl 1-Chloroheptane

22 Reaction with SOCl2

• Reaction of an alcohol with SOCl2 in the presence of a 3° amine is stereoselective. – It occurs with inversion of configuration.

OH Cl 3° amine + SOCl2 + SO2 + HCl (S)-2-Octanol Thionyl (R)-2-Chlorooctane chlorid e

OH Cl SOCl2

R R(H) Pyridine or N(Et)3 R R(H) H H

o o 1o &2o Chloride 1 &2 ROH N 23

Dehydration of ROH • An alcohol can be converted to an alkene by acid-catalyzed dehydration (a type of β- elimination). – 1° alcohols must be heated at high temperature in the presence of an acid catalyst, such as H2SO4 or H3PO4. 2° alcohols undergo dehydration at somewhat lower temperatures. – 3° alcohols often require temperatures at or only slightly above room temperature.

24 Dehydration of ROH

H2 SO4 CH3 CH2 OH CH2 =CH2 + H2 O 180°C

OH H2 SO4 + H2 O 140°C Cyclohexanol Cyclohexene

CH3 CH3 H2 SO4 CH3 COH CH3 C= CH2 + H2 O 50°C CH3 2-Methyl-2-propanol 2-Methylpropene (tert- Butyl alcohol) (Isobutylene)

Dehydration of ROH

– Where isomeric alkenes are possible, the alkene having the greater number of substituents on the double bond (the more stable alkene) usually predominates (Zaitsev rule).

OH 85% H3 PO4 CH CH CHCH 3 2 3 heat 2-Butanol

CH3 CH= CHCH3 + CH3 CH2 CH= CH2 + H2 O 2-Butene 1-Butene (80%) (20%)

26 Dehydration of ROH • Dehydration of 1° and 2° alcohols is often accompanied by rearrangement.

H2 SO4 + OH 140 - 170°C 3,3-Dimethyl- 2,3-Dimethyl- 2,3-Dimethyl- 2-butanol 2-butene 1-butene (80%) (20%) – Acid-catalyzed dehydration of 1-butanol gives a mixture of three alkenes.

H2 SO4 OH + + 140 - 170°C 1-Butanol trans- 2-butene cis- 2-butene 1-Butene (56%) (32%) (12%)

Dehydration of ROH

Step 1: Proton transfer to the -OH group gives an oxonium ion.

H H + H O rapid and O + reversible + H O H + O H

H H

Step 2: Loss of H2O gives a carbocation intermediate.

HH+ slow, rate O determining

+ H2 O

A 2° carbocation intermediate 28 Dehydration of ROH

Step 3: Proton transfer from a carbon adjacent to the positively charged carbon to water. The sigma electrons of the C-H bond become the pi electrons of the carbon-carbon double bond.

rap i d an d re v e rs i b l e + H O + + + H O H H H H H

Oxidation of 1o ROH

• Oxidation of a primary alcohol gives an aldehyde or a carboxylic acid, depending on the experimental conditions. OH O O [O] [O] CH3 -C H CH3 -C-H CH3 -C-OH H A primary An aldehyde A carboxylic alcohol acid – oxidation to an aldehyde is a two-electron oxidation. – oxidation to a carboxylic acid is a four-electron oxidation.

30 Oxidation of ROH

• A common oxidizing agent for this purpose is chromic acid, prepared by dissolving chromium(VI) oxide or potassium dichromate in aqueous sulfuric acid.

H2 SO4 CrO3 + H2 O H2 Cr O 4 Chromium(VI) Chromic acid oxide

H2 SO4 H2 O K2 Cr2 O7 H2 Cr 2 O7 2H2 Cr O 4 Potassium Chromic acid dichromate H2CrO4 31

Oxidation of 1o ROH • Oxidation of 1-octanol gives octanoic acid. – The aldehyde intermediate is not isolated.

O H2 CrO4 OH H H O, acetone 1-Hexanol2 Hexanal (not isolated) O (Chromic acid in acetone is OH the Jones oxidation) Hexanoic acid

32 Oxidation of 2o ROH • A 2° alcohol is oxidized by chromic acid to a ketone.

OH O 3+ + H CrO + Cr 2 4 acetone

2-Isopropyl-5-methyl- 2-Isopropyl-5-methyl- cyclohexanol cyclohexanone (Menthol) (Menthone)

OH O H2CrO4

R R' or Na2Cr2O7 /H2SO4 or CrO3/H2SO4 R R' 2o ROH Ketones

Oxidation of 1o ROH to RCHO • Pyridinium chlorochromate (PCC): A form of Cr(VI) prepared by dissolving CrO3 in aqueous HCl and adding pyridine to precipitate PCC as a solid. – PCC is selective for the oxidation of 1° alcohols to aldehydes; it does not oxidize aldehydes further to carboxylic acids.

pyridinium ion chlorochromate ion

- CrO3 ++HCl ClCrO3 N N H Pyrid ine Pyrid inium ch lorochromate (PCC)

PCC

34 Oxidation of 1o ROH

– PCC oxidizes a 1° alcohol to an aldehyde.

O PCC OH H Geraniol Geranial

Oxidation of 2o ROH

– PCC oxidizes a 2° alcohol to a ketone.

OH PCC O

Menthol Menthone

OH O PCC or PDC

R R' CH2Cl2 R R' 2o ROH Ketones