ALCOHOLS 1 340 Chem 1 - PowerPoint Presentation

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ALCOHOLS

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Learning

Objectives

By the end of this chapter the student will:

know the structure of alcohol

Knew the different classes of alcohols.Knew the nomenclature of Alcohols Knew the physical Properties. Knew the acidity of Alcohols. Knew the different methods of preparation of Alcohols .Knew the chemical reactions of Alcohols

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Alcohols

Structural Characteristic of Alcohols

alcohols, a class of compounds containing the OH (hydroxyl) group. Alcohols have a hydroxyl ( OH) group bonded to a

saturated

carbon atom. The alcohol carbon atom may be part of a simple alkyl group, an alkenyl or

alkynyl group, or the carbon atom may be a saturated carbon atom that is attached to a benzene ring:3

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Classification of Alcohols

Alcohols are classified as primary, secondary, or tertiary according to the classification of the carbon that bears the functional group.

The carbon atom which connected to the hydroxyl group called carbinol carbon.

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Nomenclature of Alcohols

In the IUPAC system

: selected the longest carbon chain that contains the -OH group as the parent alkane and numbered from the end closer to OH. change the suffix -

e

of the parent alkane to -oluse a number to show the location of the OH group. If there is a functional group suffix and a substituent, the functional group suffix gets the lowest possible number.For cyclic alcohols, numbering begins with the carbon bearing the OH group. If the OH group is understood to be on carbon 1 of the ring.In complex alcohols, the number for the hydroxyl group is often placed between the infix and the suffix. So the compound containing two hydroxyl groups is named as a

diol, one containing three hydroxyl groups as a triol, and so on.

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Compounds containing OH and C=C groups are often referred to as

unsaturated alcohols.

choose the chain that include them both even if this is not the longest chain. In the IUPAC system, the double bond is shown by changing the infix of the parent alkane from -an- to -

en

- and the hydroxyl group is shown by changing the suffix of the parent alkane from -

e to -ol. Numbers must be used to show the location of both the carbon-carbon double bond and the hydroxyl group. Common names derived by naming the alkyl group followed by the word alcohol

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IUPAC Name

Alcohol functional group suffix is

ol

CH

3

OH methanol

Note

Common Name

alkyl group attached to OH, plus alcohol

CH

3

OH methyl alcohol

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Chair Conformations

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Physical Properties of Alcohols

Most of the common alcohols, up to about 11 or 12 carbon atoms, are liquids at room temperature. Methanol and ethanol are free-flowing volatile liquids with characteristic fruity odors. The higher alcohols (the

butanols

through the

decanols

) are somewhat viscous, and some of the highly branched isomers are solids at room temperature. These higher alcohols have heavier but still fruity odors.

SolubilityWater and alcohols have similar properties because they all contain hydroxyl groups that can form hydrogen bonds. Several of the lower-molecular-weight alcohols as CH

3

OH,,,,C

3

H

7

OH are miscible (soluble in any proportions) with water.

The solubility decreases as the alkyl group becomes larger.

The number of hydroxyl groups

increases so

the solubility

increases

.

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Boiling Points

Alcohols have much higher boiling points than comparable ethers or hydrocarbons.

Alcohol molecules can associate with each other through hydrogen bonding, whereas those of ethers and hydrocarbons cannot.The boiling points increase with the increase of

the number

of OH groups .

the boiling point decreases with increase in branching in the alkyl group. Boiling points of 1o alcohol > 2o alcohol > 3o alcohol

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Acidity of Alcohols

Alcohols can function as both weak acids (proton donors) and weak bases (proton

acceptors).A strong base can remove the hydroxyl proton to give an alkoxide ion.Alkoxide ions are strong nucleophiles and strong bases

The order of acidity of various liquid alcohols generally is water > 1

o

>2o>3oElectron-withdrawing groups make an alcohol a stronger acid by stabilizing the conjugate base (alkoxide)The acidity decreases as the substitution on the alkyl group increases, because a more highly substituted alkyl group inhibits solvation of the alkoxide ion, decreasing the stability of the alkoxide ion and driving the dissociation equilibrium toward the left.

Alcohol weaker acids than phenol and carboxylic acidsThe basicity of alkoxide ions increases while going from primary to tertiary. This increase in basicity occurs because the conjugate base of a weak acid is strong.

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The common name of an

alkoxide

is constructed by deleting the final

yl

from the name of the alkyl group and adding the suffix oxide. In substitutive nomenclature, the suffix ate is simply added to the name of the alcohol.

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Synthesis of Alcohols

1-Synthesis of Alcohols from alkenes

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a- By acid

catalysed

hydration in accordance with Markownikoff’s rule.b-Anti-Markovnikov’s rule, ( Hydroboration-Oxidation)

C- Dihydroxylation: synthesis of 1,2-diols from alkenes

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2-Hydrolysis of alkyl halides

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3-Reaction of Grignard reagents with

aldehydes , ketones, esters and epoxides

Aldehyde

Ketone

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Ester or acid chloride ( CH

3COOCH

3 or CH3COCl)

Epoxide

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4-Reduction of Aldehydes, Ketones, Acids and Esters

by hydrogenation of the carbon–oxygen double bond.

sodium borohydride (NaBH4) reduce carbonyl groupslithium

aluminum hydride (LiAlH

4

) reduce all kind of carbonyl groupsH2/ Ni or Pt or Pd or Ru reduce all double bond19

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Reactions of AlcoholsI.

Reaction

as acids (Breaking of oxygen- Hydrogen bond CO ـــــ

H)

A. Formation of Salt

(Na, K, Mg)

B. Formation of Esters

(

Carboxylic acid ester)

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II. Reaction

involving carbon-oxygen bond

cleavage C-OHConversion of Alcohols into Alkyl Halides

The order of reactivity of alcohols is 3°> 2° >1° > methyl.

By using Hydrogen halides (

HCl, HBr, Hl) or Phosphorus tribromide (PBr3) or Thionyl chloride (SOCl

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Elimination Reactions(Dehydration of alcohols)

1. Formation of alkenes

2. Ethers Formation

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Oxidation of Alcohols to Carbonyl Compounds

Cu/ Heat or CrO3

/ Pyridine or PCC/CH2Cl2 (weak)

(STRONG):

KMnO

4, K2Cr2O7/H3O

+ , H2Cr2O

7

/H

3

O

+

, HNO

3

Tertiary alcohols have no hydrogen on their hydroxyl-bearing carbon and do not undergo oxidation readily

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