Chapter 8 Properties of Alcohols

I. NomenclatureA. Functional Groups

1) RCH2—OH = Hydroxy functional group

2) Alcohol = organic molecule having a hydroxy group

3) Replace one H of water with R: H—O—H R—O—H

4) Ether = replace both H’s of water with R: R—O—R

B. Naming Alcohols

1) Modify the alkane name by dropping –e and adding –ol

a) CH3OH = methanol

b) CH3CH2CH2CH2OH = butanol

c) R-OH = alkanol

2) Branched Alcohols

a) Name based on longest

chain containing -OH

OH

3-methylheptanol

OH

4-methly-3-propyl-2-octanol

b) Number each carbon starting from the closest to –OH

3) Cyclic Alcohols = cycloalkanols

4) When referring to –OH as a substituent it is called hydroxy

5) Simple alcohols often have common names

CH3OH = methyl alcohol

CH3CH2OH = ethyl alcohol

(CH3)2CHOH = isopropyl alcohol

(CH3)3COH = tert-butyl alcohol

1-propanolHO H

(S)-3-hexanolOH

2,2,5-trimethyl-3-heptanol

OH

OH

cyclohexanol

OH

1-methylcyclobutanol

OH

Cl

cis-3-chlorocyclopentanol

11 1

H2N OH

3-hydroxypropylamine

II. Structural and Physical Properties of AlcoholsA. Structure

1) Alcohol structures are similar to water: sp3 O atom with 2 lone pairs

2) Steric effects of the R groups slightly alter bond angles

3) Electronegativity of O shortens and strenthens the O—H bond compared to C—H

a) DHo(OH) = 104 kcal/mol

b) DHo(CH) = 98 kcal/mol

4) Electronegativity of O causes large molecular dipole moment for alcoholsR

OH

-

+

B. Hydrogen Bonding1) Hydrogen bond = weak, partial bond between a heteroatom bound H and

another heteroatom (N, O, X, S, etc…)

2) DHo(OH…..H) = 5 kcal/mol

3) The sum of the multiple H-bonds in water or alcohols increases the boiling points of these liquids relative to alkanes or haloalkanesMethane = -162o Chloromethane = -24.2o Methanol

= 65oNonpolarNo H-Bonds

PolarNo H-Bonds

PolarH-Bonds

C. Solubility

1) Alcohols have high water solubility (Like dissolves Like)

a) Polarities of water and alcohols are similar

b) Hydroxy group is Hydrophilic (water loving)

c) Alkyl group is Hydrophobic (water fearing)

d) Alkanes have no solubility in water (hydrophobic)

e) Organic molecules with polar functional groups (-OH, -NH2, -CO2H) have much higher water solubilities

2) The longer the alkyl group, the less water soluble the alcohol (more soluble in hydrocarbons—like dissolves like)

3) MeOH and EtOH are very similar to water as solvents: many salts will dissolve in them, because they are polar enough to separate the charges

III. Acid-Base Behavior of AlcoholsA. Alcohols are Amphoteric = can be acids or bases

B. Acidity of Alcohols

1) R—OH + H2O R—O- + H3O+

OH

Ka

alkoxide

][

]][[ 3

ROH

ROOHKa

ddissociate half is acidat which pHlog KapKa

2) Alcohols are relatively weak acids

3) Electronegativity of R—O- allows

it to stabilize (-) charge, but not

very well

4) Requires a strong base to fully deprotonate an alcohol

EtOH(pKa = 15.9) + OH- EtO- + H2O (pKa = 15.7)

MeOH(pKa = 15.5) + NH2- MeO- + NH3 (pKa = 35)

5) Acidity: MeOH > 2-Propanol > t-Butyl Alcohol (1o > 2o > 3o)

Steric disruption of solvation

of deprotonated product

R pKa R pKa

H2O 15.7 t-BuOH 18

MeOH 15.5 HCl -2.2

EtOH 15.9 RCO2H 4.7

i-PrOH 17.1 CH4 50

50% 50%

~ 0% 99.999%

6) Inductive Effects = transmission of charge through -bonds

a) Electronegative substituents increase an alcohol’s acidity

b) Electronegative groups remove electron density from R—O-, which stabilizes the anion. It is easier to deprotonate = more acidic.

C. Basicity of Alcohols

1) Lone pairs of electrons will accept H+ from strong acids

ROH + HA ROH2+ + A-

2) Conditions for Acid/Base behavior of Alcohols

ROH2+ ROH RO-

CCl3CH2OH > CHCl2CH2OH > CH2ClCH2OH > CH3CH2OH

weakbase

StrongConjugateAcid pKa = -2

Strong acid

Mild base Strong Base

Mild acid

IV. Synthesis of Alcohols by Nucleophilic SubstitutionA. For Industrial Alcohol Synthesis, see your book

B. Lab Scale Reactions

1) SN1 Reaction of Tertiary RX ROH

2) SN2 Reaction of Primary, Secondary RX ROH

3) Often harder to make the RX than the ROH (get RX from ROH)

4) E1/E2 products complicate these reactions

5) Use acetate (weak base) to prevent Elimination reactions

H2O

OH-

CH3CH2Br + CH3C

O

O- Na+CH3CH2O CCH3

OSN2

CH3CH2O CCH3

OOH -

CH3CH2OH + HO C

O

CH3

acetate ester

Ester Hydrolysis Acetic acid