©2015, Mohammed B. Alshammari

(Chapter 10 )Organic Chemistry, 7th Edition, L. G. Wade, Jr.

Structure and Synthesis of

Alcohols

• An alcohol is any organic compound in which the hydroxyl

functional group (–OH) is bound to a carbon.

• Abū Bakr Muhammad ibn Zakariyyā al-Rāzī (854 CE – 925 CE)

is credited with the discovery of ethanol.

• Classification of Alcohols

• 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.

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Chemical formula IUPAC Name Common name

Monohydric alcohols

CH3OH methanol wood alcohol

C2H5OH ethanol alcohol

C3H7OH propan-2-ol isopropyl alcohol, rubbing alcohol

C4H9OH butan-1-ol butanol, butyl alcohol

C5H11OH pentan-1-ol pentanol, amyl alcohol

C16H33OH hexadecan-1-ol cetyl alcohol

Polyhydric alcohols

C2H4(OH)2 ethane-1,2-diol ethylene glycol

C3H6(OH)2 propane-1,2-diol propylene glycol

C3H5(OH)3 propane-1,2,3-triol glycerol

C4H6(OH)4 butane-1,2,3,4-tetraol erythritol, threitol

C5H7(OH)5 pentane-1,2,3,4,5-pentol xylitol

C6H8(OH)6 hexane-1,2,3,4,5,6-hexol mannitol, sorbitol

C7H9(OH)7 heptane-1,2,3,4,5,6,7-heptol volemitol

Unsaturated aliphatic alcohols

C3H5OH Prop-2-ene-1-ol allyl alcohol

C10H17OH 3,7-Dimethylocta-2,6-dien-1-ol geraniol

C3H3OH Prop-2-yn-1-ol propargyl alcohol

Alicyclic alcohols

C6H6(OH)6 cyclohexane-1,2,3,4,5,6-hexol inositol

C10H19OH 2 – (2-propyl)-5-methyl-cyclohexane-1-ol menthol

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Structure of Water and

Methanol

• Oxygen is sp3 hybridized and tetrahedral.

• The H—O—H angle in water is 104.5°.

• The C—O—H angle in methyl alcohol is 108.9°.

Physical Properties of Alcohols• Alcohol are polar molecules (because of O-H and C-O).

• Alcohols have high boiling points due to hydrogen bonding between molecules.

• Lower members are colourless liquids with characteristic pleasant odour.

• Higher members are colourless and odourless solids.

• Lower members are water soluble and the solubility decreases as the molecular

weight increases (the size of the alkyl group increases).

• Alcohols, except ethanol, are poisonous.

• They are neutral in nature but act as Lewis bases due to the presence of a lone pair

of electrons on oxygen atom.

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Solubility in Water

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Acidity of Alcohols

• Acidity of Alcohols and Phenols Just like water, the

hydroxyl groups in alcohols are weakly acidic –

strong bases can generate alkoxide ions.

• pKa range: 15.5–18.0 (water: 15.7)

• Acidity decreases as the number of carbons

increase.

• Halogens and other electron withdrawing groups

increase the acidity.

• Phenol is 100 million times more acidic than

cyclohexanol!

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Table of Ka

Values

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Acidity of Phenols

The aromatic alcohol phenol is more acidic than

aliphatic alcohols due to the ability of aromatic rings

to delocalize the negative charge of the oxygen within

the carbons of the ring.

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Charge Delocalization on the

Phenoxide Ion

• The phenoxide ion has resonance stabilization since the negative charge can be delocalised over 4 atoms (3 carbons and 1 oxygen), making it more stable.

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IUPAC Nomenclature

1. Find the longest carbon chain containing the carbon with the —OH group.

2. Drop the -e from the alkane name, add -ol.

3. Number the chain giving the —OH group the lowest number possible.

4. Number and name all substituents and write them in alphabetical order.

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Naming Priority

1. Acids

2. Esters

3. Aldehydes

4. Ketones

5. Alcohols

6. Amines

7. Alkenes

8. Alkynes

9. Alkanes

10. Ethers

11. Halides

Highest ranking

Lowest ranking

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Hydroxy Substituent

• When —OH is part of a higher priority class of

compound, it is named as hydroxy.

4-hydroxybutanoic acid

also known as g-hydroxybutyric acid (GHB)

CH2CH2CH2COOH

OHcarboxylic acid

4 3 2 1

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Common Names

• Alcohol can be named as alkyl alcohol.

• Useful only for small alkyl groups.

isobutyl alcohol sec-butyl alcohol

CH3 CH

CH3

CH2OH CH3 CH

OH

CH2CH3

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Phenol Nomenclature

• —OH group is assumed to be on carbon 1.

• For common names of disubstituted phenols,

use ortho- for 1,2; meta- for 1,3; and para- for

1,4.

• Methyl phenols are cresols.

3-chlorophenol

(meta-chlorophenol)

4-methylphenol

(para-cresol)

OH

Cl

OH

H3C

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Give the systematic (IUPAC) name for the following alcohol.

Problem 1

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Formation of Alkoxide Ions

• Ethanol reacts with sodium metal to form sodium ethoxide (NaOCH2CH3), a strong base commonly used for elimination reactions.

• More hindered alcohols like 2-propanol or tert-butanol react faster with potassium than with sodium.

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Synthesis of Alcohols

1. Nucleophilic substitution of alkyl halide.

• 2. Hydration of alkenes also produce alcohols (Acid

catalyzed):

• An acid / base reaction. Protonation of the alkene to generate the more

stable carbocation. The p electrons act pairs as a Lewis base.

• Attack of the nucleophilic water molecule on the electrophilic carbocation

creates an oxonium ion.

• An acid / base reaction. Deprotonation by a base generates the alcohol

and regenerates the acid catalyst.

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3. Catalytic Hydrogenation

• Raney nickel is a hydrogen rich nickel powder that is more reactive than Pd or Pt catalysts.

• This reaction is not commonly used because it will also reduce double and triple bonds that may be present in the molecule.

• Hydride reagents are more selective so they are used more frequently for carbonyl reductions.

4. Hydroboration-oxidation (anti-Markovnikov)

• hydroboration–oxidation reaction is a two-step organic reaction that

converts an alkene into a neutral alcohol by the net addition of water

across thedouble bond.[1] The hydrogen and hydroxyl group are added

in a syn addition leading to cis stereochemistry

Chapter 10 22

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5. Grignard Reagents (Organometallic reagents)

• Formula R—Mg—X (reacts like R:- +MgX).

• Ethers are used as solvents to stabilize the complex.

• Iodides are most reactive.

• May be formed from any halide.

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Reaction with Carbonyl

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Mechanism

C

CH3

Cl

OR MgBr C

CH3

RO

+ MgBrCl

C O

Cl

H3C

MgBrR MgBr C

CH3

Cl

OR

Step 1: Grignard attacks the carbonyl forming the tetrahedral

intermediate.

Step 2: The tetrahedral intermediate will reform the carbonyl and form

a ketone intermediate.

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Mechanism continued

HOHC

CH3

R

OHRC

CH3

R

OR MgBr

C

CH3

RO

R MgBr + C

CH3

R

OR MgBr

Step 3: A second molecule of Grignard attacks the carbonyl of the

ketone.

Step 4: Protonation of the alkoxide to form the alcohol as the product.

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Addition to Ethylene Oxide

• Grignard and lithium reagents will attack epoxides (also called oxiranes) and open them to form alcohols.

• This reaction is favored because the ring strain present in the epoxide is relieved by the opening.

• The reaction is commonly used to extend the length of the carbon chain by two carbons.

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Reduction of Carbonyl

• Reduction of aldehyde yields 1º alcohol.

• Reduction of ketone yields 2º alcohol.

• Reagents:

Sodium borohydride, NaBH4

Lithium aluminum hydride, LiAlH4

Raney nickel

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Mechanism of Hydride

Reduction

• The hydride attacks the carbonyl of the aldehyde or

the ketone.

• A tetrahedral intermediate forms.

• Protonation of the intermediate forms the alcohols.

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Reduction with LiAlH4

• The LiAlH4 (or LAH) will add two hydrides to

the ester to form the primary alkyl halide.

• The mechanism is similar to the attack of

Grignards on esters.

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Reducing Agents

• NaBH4 can reduce

aldehydes and

ketones but not

esters and

carboxylic acids.

• LiAlH4 is a stronger

reducing agent and

will reduce all

carbonyls.

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Show how you would synthesize the following alcohol from compounds containing no more than five

carbon atoms.

Solved Problem 2