alcohol_phenol-fazli-in_class (2).pptx

7/28/2019 Alcohol_Phenol-fazli-IN_Class (2).pptx

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STRUCTURE OF ALCOHOLS

Structurally, alcohols and phenols may be viewed asorganic derivatives of water.

General formula: R-OH

Functional group -OH (hydroxyl group)

alcohol : R = alkyl

phenol : Ar = aromatic ring


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CLASSIFICATION OF ALCOHOLS

Primary ROH:Carbon (carbinol carbon) withOH is bonded to oneother carbon.

Secondary ROH:Carbon withOH is bonded to two other carbons.

Tertiary ROH:

Carbon withOH is bonded to three other carbons.

Aromatic alcohol or phenol:-OH is bonded to a benzene ring.


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IUPAC NOMENCLATURE/NAMES

1. Name the longest carbon chain containing theOH group. Change e (alkane) to ol.

2. Number the carbon chain to give the OH group

the lowest number. ( The hydroxyl group takesprecedence over double/triple bonds)

3. Name all the substituents and give their numbers,

as you would for an alkane/alkene


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UNSATURATED ALCOHOLS

Priority goes to the hydroxyl group; assign that carbon

the lowest number.

Use alkene or alkyne name.

4-penten-2-ol

(pent-4-ene-2-ol)

OH

Q:

Give each compounds a systematic name

OH

OH

OHCl

1-methyl-2-cyclohexen-1-ol (z)-5-octen-1-ol 1-chloro-3-ethylpent-1-en-4-yn-3-ol


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NAMING PRIORITY

1. Acids

2. Esters

3. Aldehydes

4. Ketones

5. Alcohols

6. Amines

7. Alkenes

8. Alkynes

9. Alkanes10. Ethers

11. Halides


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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

HO

O

OH

O

C H 2 O H

2-hydroxymethylcyclohexanone


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NAMING DIOLS Two numbers are needed to locate the twoOH groups.

Use -diol as suffix instead of -ol.

Glycols 1, 2 diols (vicinal diols) are called glycols.

Common names for glycols use the name of the

alkene from which they were made.

OHHO

hexane-1,6-diol or 1,6-hexanediol

1,2-propanediolpropylene glycol

OHHO


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NAMING PHENOLS

OH group is assumed to be on carbon 1.

For common names of disubstituted

phenols, use ortho- for 1,2; meta- for 1,3;

andpara- for 1,4.

Q: Give each compounds a systematic name

OHOH

NO2

OH

OH

phenol

4-ethyl-2-nitrophenol 2-methyl-4-phenyl-2-butanol phenylmethanol


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PHYSICAL PROPERTIES

Unusually high boiling points due to hydrogen

bonding between molecules.

Small alcohols are miscible in water, but

solubility decreases as the size of the alkyl group

increases; why??? alkyl group is hydrophobic.


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BOILING POINTS

Intermolecular forces involved:a) hydrogen bondingb) dipole-dipole attractions

In increasing order:

Propane < dimethyl ether < ethanol


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Q:Place the following compounds in decreasing orderof boiling points andaccount for the order.

1-pentanol, 2-methyl-2-butanol, 3-methyl-2-butanol(A) (B) (C)

Answer A > C > B

As branching increases, the shape becomes morecompact and spherical. There is less surface contactavailable for Van der Waals attractive forces betweenmolecules hence lowering the boiling points.


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SOLUBILITY IN WATER

Solubility decreases as the sizeof the alkyl group increases.

Miscible: soluble in any proportions


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Q:Arrange the following alcohols in decreasing orderof solubility in water.Explain your answer.

1-butanol, 2-methyl-1-propanol, 2-methyl-2-propanol(A) (B) (C)

Answer C > B > A

Alcohols with branched alkyl groups are more soluble

in water than alcohols with nonbranched alkyl groupswith the same number of carbons because branchingminimizes the contact surface of the nonpolar portionof the molecule.


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The longer the carbon chain

The higher the boiling point

Due to the increased Van der Waals attractive forces betweenmolecules.

The lower the water solubility

Because the non-polar part is getting longer and hence a smallerproportion of the molecule is interacting in the water.

The more spherical (branched) the molecules, the less

surface area it has.

Lower boiling point than other isomeric alcohol

Less Van der Waals attractive forces between molecules itself.

More soluble in water

The non-polar surface area is minimized.


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ROH + H2O RO-

+ H3O+

Ka= acid-dissociation constant

pKa = - log (Ka)

Lower pKa = more

acidic


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pKa range: 15.5-18.0 (pKa water = 15.7)

Acidity decreases as alkyl group increases.

Halogens increase the acidity. Why???

(inductive effect)

CH3CH2OH + H2O CH3CH2O-

+ H3O+

CH2ClCH2OH + H2O CH2ClCH2O-

+ H3O+

(stabilised by Cl)

pKa: 15.9

pKa: 14.3

Q: Predict which member of each pair will be moreacidic

a. Propanol or tert-butyl alcohol

b. 2-chloropropan-1-ol or 3-chloropropan-1-ol


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Phenol (pKa=10.0) is 100 million times more acidic than

cyclohexanol (pKa=18.0). Why???

pKa ----- acidity

Phenols react with NaOH solutions (but alcohols do not),forming salts that are soluble in dilute aqueous solution.


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Phenoxide ion is more stable; delocalization of theve charge via resonance around the benzene ringmake it stable; hence increase the acidity.

O

noresonanc


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Effect of electron-withdrawing groups:

- NO2 , - COOH , - COOR , - SO3H , - CHO , - CN, - X

Acidity of Phenols

Effect of electron-donating groups:

- NH2 ,

- OH , - OR , - NHCOR , - C6

H6

,

- R

Acidity increase; -ve charge is delocalize to the groups.

Acidity decrease; -ve charge is localize to oxygen atom(ROH).


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O

N

O O

O

N

O O

O

N

O O

O

N

O O

O

N

O O

p-nitrophenol (pKa= 7.6) is almost 1000 times more acidic than phenol itself.

Why???

Further delocalisation ofve charge of the phenoxide ion

to the oxygen of the nitro group


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Q: Isp-hydroxybenzaldehyde more acidic or lessacidic than phenol?

Electron-withdrawing substituentsmake thephenol more acidic by stabilizing the

phenoxide anion

Electron-donating substituentsmake thephenol less acidic by destabilizing the anion

Answer: more acidic

alcohol_phenol-fazli-in_class (2).pptx

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