Download Lecture (8)

Lecture (8)
Hydrocarbons derivatives:
The vast majority of organic molecules contain elements in addition to
carbon and hydrogen. However, most of these substances can be viewed
as hydrocarbons derivatives; e.g.: alcohols and phenols, aldehydes and
ketons and carboxylic acids.
V-Alcohols
1-Introduction:
Alcohols are compounds of the general formula R-OH, where R is any
Alkyl group; the group may be primary, secondary or tertiary; it may be
open chain or cyclic, it may contain a double bond, or an aromatic ring.
2-Classification of alcohols:
Alcohols are classified as primary (1°), secondary (2°) or tertiary (3°);
depending on the number of carbon substituents bonded to the hydroxylbearing carbon:
H
R
R
R
C
OH
H
Primary alcohol (1°)
H
C
OH
R
R
secondary alcohol (2°)
C
OH
R
tertiary alcohol (3°)
3-Nomenclature of alcohols:
Alcohols are named by many system using common names, carbinol
system or IUPAC system (the name of hydrocarbon from which they are
derived, followed by the suffix "-OL".
IUPAC system:
1- Select the longest carbon chain containing the hydroxyl group, and
replace the –e ending of the corresponding alkane with "OL"
2-Number the carbon; begin at the end nearer to the hydroxyl group.
3-Number all substituents according to their position; and write in
alphabetical order.
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Examples:
OH
(1)-CH3 C CH2CH2CH3
1
2
3
4
2-methyl-2-pentanol (IUPAC)
5
CH3
CH3
(2)-CH3
C
OH
2-methyl-2-propanol (IUPAC)
Tert-butyl alcohol (common)
Tri methyl carbinol (carbinol)
CH3
Alcohols containing two hydroxyl groups are called { Dihydric
alcohols};[GLYCOLS]:
Examples:
CH2 ─ CH2
1,2-Ethandiol (IUPAC)
OH
Ethylene glycol (common)
OH
CH2 CHCH3
1,2-Propandiol(IUPAC)
OH OH
Propylene glycol (common)
Alcohols containing three hydroxyl groups are called {Trihydric
alcohols}:
OH OH OH
CH2- CH- CH2
Glycerine (1,2,3-trihydroxyPropane)
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4-Prepatration of Alcohols:
A-Reduction of carbonyl compounds (ketones or aldehydes)
O
H2/Pt
(1)-CH3 CH2 C ─H
CH3CH2CH2─OH
Propanal
High pressure
n-propyl alcohol (1°)
CH3
CH3
H2/Pt
(2)-CH3─C=O
Acetone
CH3─CH─OH
Isopropyl alcohol (2°)
B-Hydration of Alkenes:
H
H
CH3CH=CH2 cold H2SO4 CH3─ C─CH3 H2O CH3─C─CH3+H2SO4
∆
OSO3H
OH
Iso propyl alcohol
C-By Grignard reactions with aldehydes or ketones:
RMgX/H2O
(1)-HCHO
RCH2OH + MgXOH
Formaldehyde
(1°)
RMgX/H2O
(2)-R'CHO
R'RCHOH + MgXOH
Aldehyde
(2°)
RMgX/H2O
(3)-R'R"CO
R'R"RCOH + MgXOH
Ketone
(3°)
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5-Physical Properties of Alcohols:
Alcohols, like water, are both weakly basic and weakly acidic.
Alcohols contain the very polar –OH group. This group contains
hydrogen attached to the very electronegative element OXYGEN, and
therefore permits hydrogen bonding R─O…..H-O-R
H
The boiling point of alcohols is higher than hydrocarbons of the same
molecular weight. Lower alcohols are miscible with water.
6-Chemical properties of alcohols:
The chemical properties of an alcohol are determined by its functional
group, the hydroxyl group reactions of an alcohol can involve the
breaking of either of two bonds, C─OH bond, with removal of the (OH)
group or the (O-H) bond with removal of (-H).Either kind of reaction can
involve substitution, in which a group replaces the (-OH) or (-H), or
elimination, in which a double bond is formed. Difference in the structure
of the group cause differences in reactivity and some of the more
important reactions of alcohols are listed below:
A-Reactions based on the substitution of the hydroxyl group (R─OH
bond cleavage):
Reaction with hydrogen halides:
ROH + HX
R-X + H2O
e.g.:
HCL/ZnCL2
CH3CH2CH2CH2CH2OH
CH3CH2CH2CH2CH2CL +H2O
n-Pentylalcohol
Heat
n-Pentylchloride
Reactivity towards ROH (HX)
HI>HBr>HCL
Reactivity towards HX (ROH)
3°>2°>1°
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Examples:
25°
(1)- (CH3)3COH + HCL
Tertiary Alcohol (3°)
(CH3)3 CCL + H2O
ZnCL2
(2)- (CH3)2CHOH + HCL
Secondary alcohol (2°)
(CH3)2CHCL + H2O
ZnCL2/heat
(3)- (CH3)CH2OH + HCL
Primary alcohol (1°)
CH3CH2CL + H2O
B-Reactions based on the substitution of the hydrogen of the
hydroxyl group (RO-H bond cleavage):
1-Reaction with active metals (as acid)
RO-H + M
RO-M+ + 1/2 H2 (M=Na,K,Mg,AL)
Reactivity of ROH:
The rate of reaction is higher with primary and decreases with
secondary then to tertiary alcohol.
Na
CH3CH2OH
CH3CH2O-Na+ + 1/2 H2
Ethyl alcohol
sodium ethoxide
2-Ester formation
CH3CH2O-H + CH3COOH
Ethyl alcohol acetic acid
:
H
+
H3C-COOCH2CH3 + H2O
ethylacetate
C-Dehydration:
Elimination of water to yield alkenes:
Conc.H2SO4(-H2O)
─C ─C─
─C = C─
(Reactivity of ROH 3°>2°>1°)
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D-Oxidation of alcohols :( Differentiation between alcohols)
The oxidation of an alcohol involves the loss of one or more hydrogen (αhydrogens) from the carbon bearing the –OH group. The kind of product
that is formed depends upon how many of this α-hydrogen the alcohol
contains, that is, upon whether the alcohol is primary, secondary or
tertiary.
(1)-Primary alcohol contains two α-hydrogens and can lose one of them
to form an aldehyde.
KMnO4
RCH2OH
RCHO
RCOOH
Alcohol (1°)
aldehyde
carboxylic acids
(2)-Secondary alcohol can lose its only α-hydrogen to form a ketone
K2Cr2O7
R2CHOH
R2CO
Alcohol (2°)
(3)-Tertiary alcohol contains no α-hydrogen and is not oxidized.
KMnO4
R3COH
no oxidation
Oxidation of primary alcohols to carboxylic acids is usually by potassium
permanganate. Oxidation of secondary alcohols to ketone by potassium
dichromate.
7-Relative acids:
The alkyl group makes an alcohol less acidic than water, the bigger the
alkyl group, the less acidic the alcohol.
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