Transcript Chapter 8 Properties of Alcohols I. Nomenclature

Chapter 8 Properties of Alcohols
I.
Nomenclature
A.
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)
OH
Branched Alcohols
a) Name based on longest
chain containing -OH 3-methylheptanol
OH
4-methly-3-propyl-2-octanol
b)
Number each carbon starting from the closest to –OH
OH
1-propanol
3)
HO H
(S)-3-hexanol
OH
2,2,5-trimethyl-3-heptanol
Cyclic Alcohols = cycloalkanols
OH
Cl
1
1
cyclohexanol
4)
5)
OH
1-methylcyclobutanol
1
OH
cis-3-chlorocyclopentanol
When referring to –OH as a substituent it is called hydroxy
3-hydroxypropylamine
Simple alcohols often have common names
CH3OH = methyl alcohol
H2N
OH
CH3CH2OH = ethyl alcohol
(CH3)2CHOH = isopropyl alcohol
(CH3)3COH = tert-butyl alcohol
II.
Structural and Physical Properties of Alcohols
A.
Structure
1) Alcohol structures are similar to water: sp3 O atom with 2 lone pairs
2)
3)
Steric effects of the R groups slightly alter bond angles
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 alcohols
R
H
O
+
-
B.
Hydrogen Bonding
1) 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 haloalkanes
Methane = -162o
Chloromethane = -24.2o
Methanol = 65o
Polar
Polar
Nonpolar
No H-Bonds
H-Bonds
No H-Bonds
C.
Solubility
1) Alcohols have high water solubility (Like dissolves Like)
a) Polarities of water and alcohols are similar
OH
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 Alcohols
A.
B.
Alcohols are Amphoteric = can be acids or bases
Acidity of Alcohols
Ka
1) R—OH + H2O
R—O- + H3O+
alkoxide
[ H 3O  ][ RO  ]
Ka 
[ ROH ]
pKa   log Ka  pH at which acid is half dissociate d
2)
3)
4)
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
Alcohols are relatively weak acids
Electronegativity of R—O- allows
it to stabilize (-) charge, but not
very well
Requires a strong base to fully deprotonate an alcohol
EtOH(pKa = 15.9) + OH-  EtO- + H2O (pKa = 15.7)
50%
50%
MeOH(pKa = 15.5) + NH2-  MeO- + NH3 (pKa = 35)
~ 0%
5)
99.999%
Acidity: MeOH > 2-Propanol > t-Butyl Alcohol (1o > 2o > 3o)
Steric disruption of solvation
of deprotonated product
6)
Inductive Effects = transmission of charge through s-bonds
a) Electronegative substituents increase an alcohol’s acidity
CCl3CH2OH
b)
C.
>
CHCl2CH2OH
>
CH2ClCH2OH
>
CH3CH2OH
Electronegative groups remove electron density from R—O-, which
stabilizes the anion. It is easier to deprotonate = more acidic.
Basicity of Alcohols
1) Lone pairs of electrons will accept H+ from strong acids
ROH + HA  ROH2+ + Aweak
Strong
base
Conjugate
Acid pKa = -2
2)
Conditions for Acid/Base behavior of Alcohols
ROH2+
Mild base
Strong acid
ROH
Strong Base
Mild acid
RO-
IV. Synthesis of Alcohols by Nucleophilic Substitution
A.
B.
For Industrial Alcohol Synthesis, see your book
Lab Scale Reactions
H2O
1) SN1 Reaction of Tertiary RX
ROH
OH2) 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
O
CH3CH2Br +
-
+
CH3C O Na
acetate
O
CH3CH2O CCH3
OH
SN 2
O
CH3CH2O CCH3
ester
O
-
CH3CH2OH + HO C CH3
Ester Hydrolysis
Acetic acid