ORGANIC CHEMISTRY
CHM 207
CHAPTER 1:
INTRODUCTION TO ORGANIC
CHEMISTRY
NOR AKMALAZURA JANI
SUBTOPICS
• Characteristic features of organic
compounds.
• Electronegativity and chemical bonds.
• Isomerism.
• Reaction of organic compounds.
ORGANIC CHEMISTRY
• Organic chemistry:
-The branch of chemistry that deals with carbons
compounds.
• Also contain element such as O, N, P, S and halogen (F,
Cl, Br, I)
• ‘Organic’ – derived from living organisms
• Study of compounds extracted from living organisms
and their natural products.
• Examples: sugar, starch, urea, waxes, carbohydrates,
fats and etc
• Human are composed of organic molecules – proteins
in skin, lipid in cell membranes, glycogen in livers and
the DNA in the nuclei of cells.
• Chemistry of carbon:
- Two stable isotops (13C and 12C)
- electron configuration: 1s2 2s2 2p2
- four valence electrons
- can form more compounds than any other
element
- able to form single, double and triple carboncarbon bonds, and to link up with each other in
chains and ring structures
Characteristic features of organic
compounds
•
Bonding hybridization theory
i) sp hybrid orbitals
- combine a s orbital and a p orbital on the same atom
- hybrid orbitals give a bond angle of 180o
- linear bonding arrangement
- example: acetylene (contains carbon triple bond)
-
Bonding in Acetylene, C2H2
180o
0 lone pairs on central atom
Cl
Be
Cl
2 atoms bonded to central atom
• sp2 hybrid orbitals
- when an s orbital combines with two p orbitals, it will
formed three hybrid orbitals and oriented at 120o angles to
each other.
- called sp2 hybrid orbitals (composed of one s orbital and
two p orbitals).
- the 120o arrangement is called trigonal geometry.
121.7o
116.6o
Ethylene, close to 120o
• sp3 hybrid orbitals
- when an s orbital combines with three p orbitals, it will
formed four hybrid orbitals and oriented at 109.5o angles to
each other.
- called sp2 hybrid orbitals (composed of one s orbital and
three p orbitals).
- the arrangement is called tetrahedral.
• Hydridization of other atoms: Nitrogen and oxygen
- type of hybridization: sp3
- bond angle for N-H : 107.3o
- bond angle for O-H : 104.5o
Valence shell electron pair repulsion (VSEPR)
model:
Predict the geometry of the molecule from the
electrostatic repulsions between the electron
(bonding and nonbonding) pairs.
Class
AB2
# of atoms
bonded to
central atom
2
# lone
pairs on
central atom
0
Arrangement of
electron pairs
Molecular
Geometry
linear
linear
B
A = central atom
B = surrounding atom
E = lone pair on A
B
VSEPR
Class
AB2
AB3
# of atoms
bonded to
central atom
2
3
# lone
pairs on
central atom
Arrangement of
electron pairs
Molecular
Geometry
0
linear
linear
0
trigonal
planar
trigonal
planar
Boron trifluoride (BF3)
VSEPR
Class
AB2
# of atoms
bonded to
central atom
# lone
pairs on
central atom
2
0
linear
linear
trigonal
planar
trigonal
planar
AB3
3
0
AB4
4
0
Arrangement of
electron pairs
tetrahedral
Molecular
Geometry
tetrahedral
VSEPR
Class
AB2
# of atoms
bonded to
central atom
# lone
pairs on
central atom
2
0
linear
linear
trigonal
planar
trigonal
planar
Arrangement of
electron pairs
Molecular
Geometry
AB3
3
0
AB4
4
0
tetrahedral
tetrahedral
0
trigonal
bipyramidal
trigonal
bipyramidal
AB5
5
VSEPR
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB2
2
0
linear
linear
AB3
3
0
trigonal
planar
trigonal
planar
AB4
4
0
tetrahedral
tetrahedral
trigonal
bipyramidal
octahedral
Class
Arrangement of
electron pairs
AB5
5
0
trigonal
bipyramidal
AB6
6
0
octahedral
Molecular
Geometry
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
Arrangement of
electron pairs
Molecular
Geometry
trigonal
planar
bent
AB3
3
0
trigonal
planar
AB2E
2
1
trigonal
planar
VSEPR
Class
AB4
# of atoms
# lone
bonded to
pairs on
central atom central atom
4
0
Arrangement of
electron pairs
Molecular
Geometry
tetrahedral
tetrahedral
AB3E
3
1
tetrahedral
trigonal
pyramidal
AB2E2
2
2
tetrahedral
bent
VSEPR
Class
AB5
# of atoms
bonded to
central atom
# lone
pairs on
central atom
5
0
trigonal
bipyramidal
trigonal
bipyramidal
distorted
tetrahedron
T-shaped
Arrangement of
electron pairs
AB4E
4
1
trigonal
bipyramidal
AB3E2
3
2
trigonal
bipyramidal
AB2E3
2
3
trigonal
bipyramidal
Molecular
Geometry
linear
VSEPR
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB6
6
0
octahedral
octahedral
AB5E
5
1
octahedral
square
pyramidal
AB4E2
4
2
octahedral
square planar
Class
Arrangement of
electron pairs
Molecular
Geometry
Class
Molecular Geometry
AB2
linear
AB3
AB2E
trigonal planar
bent
AB4
AB3E
AB2E2
tetrahedral
trigonal pyramidal
bent
AB5
AB4E
AB3E2
AB2E3
trigonal bipyramidal
distorted tetrahedron
T-shaped
linear
AB6
AB5E
AB4E2
octahedral
square pyramidal
square planar
ELECTRONEGATIVITY AND CHEMICAL BONDS
• Electronegativity: a measure of the force of an atom’s
attraction of electrons that is shares in a chemical bond with
another atom.
• Electronegativity is used to estimate the degree of ionic or
covalent character in a chemical bond.
• A non polar covalent bond is one in which the difference in
electronegativity between the bonded atoms is 0.4 or less.
- example: bond between C and H is classified as nonpolar
covalent because the difference in electronegativity
between two atoms is 0.4 unit on the Pauling scale.
• A polar covalent bond is one in which the difference in
electronegativity between two atoms is between 0.5 and
1.8.
- example: bond between H-Cl (electronegativity between
two atoms is 0.9 unit)
CH3 CH3
ethane
nonpolar
CH3 NH2
CH3 OH
methylamine
methanol
increasing polarity
CH3 Cl
chloromethane
H3CNH3+ Clmethylammonium chloride
ionic
e- poor
electron poor
region
H
d+
electron rich
region
e- rich
H
F
d+
d-
F
d-
cross arrow = indicate the direction of bond polarity
The Electronegativities of Common Elements
ISOMERISM
• Isomers: organic compounds that have same
molecular formula but different arrangements of
atoms.
• Isomerism: the existence of two or more organic
compounds with the same molecular formula but
different arrangements of atoms.
• Two types of isomerism:
i) constitutional isomerism
ii)stereoisomerism
CONSTITUTIONAL (STRUCTURAL)
ISOMERISM
• Molecules that have same molecular formula but
have different structural formulae.
• Subdivided into 3 different categories:
i) Chain isomerism
ii)Positional isomerism
iii)Functional group isomerism
Chain isomerism
• Chain isomers are those which differ in the structure of
their carbon chains, differ in the length of their straight
chains or branch.
• Example:
- alkane with molecular formula C4H10 has 2 chain
isomers
CH3CH2CH2CH3
butane
(straight chain)
CH3CHCH3
CH3
2-methylpropane
(branched chain)
- alcohols with the formula C4H9OH
CH3CH2CH2CH2OH
butanol
(straight chain)
CH3CHCH2OH
CH3
2-methyl-1-propanol
(branched chain)
• Possess the same functional group, belong to the same
homologous series.
• Different physical properties.
• Similar chemical properties.
Positional isomerism
• Have the same carbon skeleton and belong to the same
homologous series, but differ position of the functional group.
• Similar chemical properties because have the same functional
group.
• Different physical properties.
• Examples:
i) bromoalkanes with the molecular formula C3H7Br
H H H
H C C C Br
H H H
1-bromopropane
(boiling point: 70.8oC)
H H H
H C C C H
H BrH
2-bromopropane
(boiling point: 59.4oC)
ii) Alcohols with the molecular formula C3H7OH
H H H
H C C C OH
H H H
H H H
H C C C H
H OH H
1-propanol
2-propanol
iii) Alkenes with the molecular formula C5H10
CH3CH2CH2CH
1-pentene
CH2
CH3CH2CH
CHCH3
2-pentene
• Aromatic compounds.
- If two bromine atoms replace two hydrogen atoms to
form disubstituted benzene, three isomers can be
formed.
Br
Br
Br
Br
Br
1,2-dibromobenzene
1,3-dibromobenzene
Br
1,4-dibromobenzene
Functional Group Isomerism
• Isomers which have the same molecular formula but
contain different functional groups.
• Same molecular formula but belong to different
homologous series.
• Different chemical and physical properties.
Examples
Types of
compounds
Alkene and
cycloalkane
Molecular
formula
Functional group isomers
C5H10
CH3CH2CH2CH CH2
1-pentene
Alcohol and
ether
C3H8O
Aldehyde and
ketone
C3H6O
Carboxylic acid
and ester
C3H6O2
CH3CH2CH2OH
1-propanol
CH3CH2CHO
propanal
CH3CH2COOH
propanoic acid
cyclopentane
CH3OCH2CH3
methoxyethane
CH3COCH3
propanone
CH3COOCH3
methyl ethanoate
HCOOC2H5
ethyl methanoate
STEREOISOMERISM
• Isomers with same structural formula but they have
different spatial arrangements of their atoms.
• Divided into two categories:
i) cis-trans isomerism
ii)optical isomerism
cis-trans (geometrical) isomerism
• Occurs in compounds in which free rotation is prevented by the
presence of a carbon-carbon double bond or a cyclic structure which
hinders or obstructs the rotation of a C-C single bond in the ring.
• Known as geometrical isomerism.
• Same structural formula, differ only in how the atoms or groups are
orientated in space.
• Example:
- alkenes: 2-butene exists has two cis-trans isomers.
H3C
CH3
C C
H
H
cis-2-butene (bp: 4o)
H
CH3
C C
H
H 3C
trans-2-butene (bp: 1o)
(two methyl groups are on the (two methyl groups are on the
same side of the double bond) opposite side of the double bond)
cis-trans (geometrical) isomerism
• Have same functional group, thus have same chemical properties.
• Have different physical properties as a result of the different spatial
arrangements of the groups.
• For example:
- boiling point of cis-2-butene is 3.7oC but boiling point of trans-2butene is 0.9oC.
• cis isomers have higher bp compared to trans isomer due to more
polarised nature.
• trans isomer have higher melting points
- more linear structure that enables them to form a more denselypacked crystalline structure.
CH3
C C
H
CH3
H
CH3
H
C C
H
CH3
cis-2-butene
trans-2-butene
bp: 3.7oC
bp: 0.9oC
μ = 0.33 D
μ=0D
cis-trans (geometrical) isomerism
• Cannot occur if one of the carbons atoms in the double bond or the ring
structure has two identical atoms or groups.
• Example:
- 1,2-dichloroethene shows cis-trans isomerism but 1,1-dichloroethene
and 1,1,2-trichloroethene do not show cis-trans isomerism.
Cl
Cl
H
Cl
C C
C C
H
H
cis-1,2-dichloroethene
H
Cl
trans-1,2-dichloroethene
exhibit cis-trans isomerism
H
Cl
C C
Cl
H
1,2-dichloroethene
Cl
Cl
C C
H
Cl
1,1,2-trichloroethene
do not exhibit cis-trans isomerism
• Examples of cyclic compounds that show cis-trans isomerism:
H
H
CH3
CH3
cis-1,4-dimethylcyclohexane
H H
CH3 CH3
cis-1,2-dimethylcyclohexane
Cl
Cl
H
CH3
cis-1,3-dichlorocyclohexane
H
trans-1,4-dimethylcyclohexane
H CH3
CH3 H
trans-1,2-dimethylcyclohexane
H
Cl
Cl
H
H
CH3
H
trans-1,3-dichlorocyclohexane
Optical isomerism
• Optical activity is the ability of certain crystals or solutions of certain
substances to rotate the plane of plane-polarised light.
• The substances are said to be optically active.
• Optical isomers: optically active substances which possess the same
structural formula but differ in their effect on plane-polarised light.
• Isomers which rotates the plane of polarisation to the right (in
clockwise direction) – dextrorotatory isomer or the (+)isomer.
• Isomer that rotates the plane of polarisation to left (anti-clockwise
direction) – laevorotatory or the (-)isomer.
Dextrorotatory (+)
laevorotatory (-)
CHIRALITY
• Chiral:
- molecule that have different mirror image (nonsuperimpose).
- must have an enantiomers.
• Achiral:
- a molecule that do not have chiral carbon
- identical with its mirror image
- optically inactive
- not chiral
same compound
different compounds
HH
HH
Cl H
H Cl
Cl Cl
Cl Cl
H Cl
Cl H
cis-1,2-dichlorocyclopentane
(achiral)
trans-1,2-dichlorocyclopentane
(chiral)
CHIRALITY CENTRE AND ENANTIOMERS
• An organic molecule will exhibit optical isomerism or optical activity if
it contains at least one chiral carbon atom.
• Chiral carbon atom or asymmetric carbon atom (C*): carbon atom
attached to four different atoms or groups.
• Enantiomers / optical isomers:
- A molecule that contains an asymmetric carbon atom and have a
mirror image that cannot be superimposed on it.
- have same structural formula but different spatial arrangement of the
atoms.
- optically active.
• Diastreoisomers:
- stereoisomers that are not mirror image of each other.
- include cis-trans isomers.
mirror plane
CH3
C*
H
C2H5
OH
CH3
C2H5 C*
HO
H
C* = chiral carbon
optical isomerism of 2-butanol (enantiomers)
Write the expanded formula for each of these
molecules and mark the chiral carbon or
stereogenic centre in each of the molecule with
an asterisk (*):
a) 2-bromopentane
b) 1,2-dibromobutane
c) 4-ethyl-4-methyloctane
Isomerism
(same molecular
formula)
Constitutional isomerism
(different structural formula;
different spatial
arrangement of atoms)
Chain
isomerism
Positional
isomerism
Stereoisomerism
(same structural formula;
different spatial
arrangement of atoms)
Functional
group
isomerism
cis-trans
isomerism
Optical
isomerism
THREE-DIMENSION (3D) FORMULA
H
C
Cl
H
H
Cl
Cl
C H
Cl
The 3-dimensional line-wedge-dash structure and the expanded
structural formula of dichloromethane (CH2Cl2)
represents the covalent bond coming out of the plane of the paper
represents the bond behind the plane of the paper
represents the covalent bond lying in the plane of the paper
RACEMATE
• Racemate or racemic mixture:
- an equimolar mixture (+) and (-) isomers.
• Does not rotate the plane of polarised light (optically inactive).
• Example:
50% (+)-gliseraldehyde + 50% (-)-gliseraldehyde = (±)-gliseraldehyde
Reaction of Organic Compounds
• Covalent bond cleavage
- during the organic reaction, chemical bonds have to be broken in
order that new compounds may be formed.
- the breaking of chemical bonds is called fission.
- When organic compounds react, their bonds can split up in the
following ways:
XY
X
XY
Y
XY
X
X
X
X
X
XY
Y
X
X
XY
X
X
XY
Y
X
X
Y
Y
Y
Y
Y
Y
Y
Y
Y
- Each atom retains one of the electrons present
in the covalent bond.
- known as homolytic fission / homolytic
cleavage
- either X or Y retains both electrons
- known as heterolytic fission / heterolytic
cleavage
HOMOLYTIC FISSION
(RADICAL CLEAVAGE)
• Homolytic fission: two shared electrons of the covalent bond
are split equally between the two atoms joined by the bond.
• Also known as homolysis.
• One electrons goes to X and the others goes to Y.
• The resulting species is called free radicals.
XY
XY
X
Y
Free radicals
X
Y
• Examples of free radicals:
i) hydrogen radical, H•
ii)chlorine radical, Cl•
iii)methyl radical, •CH3 or CH3•
iv)ethyl radical, •CH3CH2 or CH3CH2•
• Free radicals are reaction intermediates.
• They are very reactive and many may only exist for a split
second.
• The movement of a single electron in homolytic fission is shown
by a curved half-arrow.
C
CI
C
CI
• Occurs in reactions which take place in the gas phase or in nonpolar solvents.
• The reactions are often catalysed by sunlight, ultraviolet light or
high temperatures.
HETEROLYTIC FISSION (IONIC CLEAVAGE)
• Heterolytic fission: the breaking of a covalent bond in which
both electrons remain on one of the atoms.
• Known as heterolysis.
• Produces positive and negative ions.
• The breaking of the bond (C-Y) can happen in 2 ways,
depending on the electronegativities of the carbon atom and
the atom Y.
C
C Y
heterolysis
cation
C
anion
Y
(Y is more electronegative than C)
anion
Y
cation
(C is more electronegative than Y)
• Carbonium ion or carbocation:
- an organic ion which contains a carbon atom with a positive charge (C+)
- electrophiles (lewis acid)
• Carbanion:
- an organic ion which contains a carbon atom with negative charge (C-)
- has a lone pair of electrons
• Heterolytic fission is more common where a bond is already polar.
• For example:
H
δδ+
H C Br
H
Bromomethane has a polar
C-Br bond
H
Br
H C
H
methy carbocation
δ+ = low electron density
δ- = high electron density
THE EFFECT OF ALKYL GROUPS ON
THE STABILITY OF FREE RADICALS,
CARBOCATIONS AND CARBANIONS
• Free radicals can be designated as primary (1o), secondary (2o)
and tertiary (3o).
• For example:
CH3
H
CH3 C
CH3
H
CH2 C
H
propyl radical (primary)
CH3
1-methylethyl or isopropyl radical
(secondary)
CH3
C
CH3
tert-butyl radical (tertiary)
• Free radicals are stabilized by alkyl groups that donate to the
trivalent carbon atom.
H
H CH
methyl radical
H
R C H
primary
radical
R
R CH
R
R C R
secondary tertiary
radical
radical
stability increases
• A secondary carbocation is more stable than primary
carbocation due to the electron donating effect of alkyl groups.
• The greater the number of the alkyl groups on the carbon atom
that bears the positive charge, the greater its stability.
H
H CH
R
R C R
H
R CH
R
R CH
primary
carbocation
secondary tertiary
carbocation carbocation
stability of the carbocation increases
• The alkyl group decreases the stability of a carbanion.
• The greater the number of the alkyl groups on the carbon atom
that bears the negative charge, the less stable the carbanion.
H
H CH
H
R CH
R
R CH
primary
carbanion
secondary
carbanion
R
R C R
tertiary
carbanion
stability of the carbanion decreases
ELECTROPHILE AND
NUCLEOPHILE
• Reagents in organic reactions can be classified either electrophiles
or nucleophiles depending on whether they attack regions of high
electron density (δ-) or regions of low electron density (δ+).
• regions of low electron density (δ+) also known as electrondeficient sites.
• Electrophiles:
- a species (ion or molecule) which attacks a negatively charged
carbon atom by accepting an electron pair.
- Lewis acid
-positive ions or molecule that have an electron-deficient atom
(δ+).
- known as electrophilic reagents
• Nucleophile:
- a species (ion or molecule) that attacks a positively
charged carbon atom by donating an electron pair to form a
dative covalent bond.
- known as nucleophilic reagents
- Lewis bases
- negative ions or molecules that have at least one lone pair
of electrons.
Examples of nucleophiles and electrophiles
nucleophiles
(donors of electron pairs)
electrophiles
(acceptors of electron pairs)
OH, RO
H+ (or H3O+), NO2+ (nitronium ion)
CI, Br, I
Cl2, Br2
C
+
(carbanion)
C
NH3, RNH2
H2O
Reducing agents
CN-
(carbocation)
RN2+
R-OH
C
C
BF3
AlCl3
FeBr3, ZnCl2
Oxidising agents
Cl+, Br+, I+
• Example:
H
CH3 N
H
nucleophile
δ+
CH3CH2 Cl
electrophile
δ-
H
CH3 N CH2CH3
H
Cl
TYPES OF REACTIONS
•
•
•
•
•
Addition reactions with electrophilic reagents
Addition reactions with nucleophilic reagents
Substitution reactions with electrophilic reagents
Substitution reactions with nucleophilic reagents
Elimination reactions
ADDITION REACTIONS
• Addition reaction: a reaction in which an unsaturated molecule
becomes saturated by the addition of a molecule across a
multiple bond (C=C in alkenes, -C≡C- in alkynes, C=O in aldehydes
and ketones)
• Addition reactions to double bonds have these 3 characteristics:
i) the π bond of the double bond is broken and two single bonds
are formed
ii) only one product is obtained at the end of the reaction
iii) the product obtained is a saturated organic compound
C
C
unsaturated
X Y
addition reaction
C C
X Y
saturated
Addition reactions
• Addition reactions can be classified as:
i) electrophilic addition reactions
ii)nucleophilic addition reactions
Electrophilic addition reactions
1) Addition reactions with electrophilic reagents.
• Examples:
i) Reaction of propene with bromine to give 1,2dibromopropane.
-Bromine acts as the electrophile
H
CH3 C
H
CH
propene
δ+
δBr Br
electrophile
H H
CH3 C C H
Br Br
1,2-dibromopropane
or
C
C
alkene
X X
electrophile
X = halides (Cl, Br or I)
C
X
C
X
ii) Reaction of ethene with hydrogen bromide (HBr):
H
H
H C
C H
ethene
H Br
H
H C
H
H
C H
Br
electrophile
or
C
C
alkene
H X
electrophile
X = halides
HX = HCl or HBr or HI
C
H
C
X
2) Addition reactions with nucleophilic reagents.
• Examples:
i) Aldehydes and ketones undergo nucleophilic addition
hydrogen cyanide (HCN) to form cyanohydrins.
H
CH3 C
O
H
CH3 C OH
CN
HCN
ethanal
(an aldehyde)
nucleophile
cyanohydrin
Mechanism for formation of cyanohydrin
Step 1: Cyanide addition
O
C
R R'
ketone or aldehyde
C N
Step 2: Protonation
O
R C R'
C N
intermediate
H C N
OH
R C R'
C N
cyanohydrin
SUBSTITUTION REACTIONS
• A reaction in which one atom or group replaces another atom
or group in a molecule.
• Example: the conversion of an alcohol into haloalkane
CH3CH2OH
PCl5
CH3CH2Cl
POCl3
• The hydroxyl group (-OH) in the alcohol molecule is replaced by the
chlorine atom.
• Three types of substitution reactions:
i) electrophilic substitution reactions
ii)nucleophilic substitution reactions
iii)free radical substitution reactions
HCl
1) Substitution reactions with electrophilic reagents
•
A reaction where electrophiles attack an aromatic ring
and replace one of the hydrogen atoms on the ring in a
reaction.
H
+
E
an electrophile
E
H+
•
Aromatic compounds such as benzene (C6H6), methylbenzene
(C6H5CH3), phenol (C6H5OH), and phenylamine (C6H5NH2)
undergo electrophilic substitution with chlorine under suitable
conditions.
•
For example:
i) chlorination of benzene
- benzene reacts with chlorine gas in the presence of FeCl3
(Lewis acid catalyst) to form chlorobenzene and hydrogen
chloride.
H
Cl
Cl2
FeCl3
benzene electrophile
HCl
chlorobenzene hydrogen chloride
ii) bromination of benzene
H
Br
Br2
FeBr3
benzene electrophile
HBr
bromobenzene hydrogen bromide
iii) nitration of benzene
NO2
H
HNO3
H2SO4
H2O
nitrobenzene
+
NO3 (nitronium ion) = electrophile
2) Substitution reactions with nucleophilic reagents
- any reaction in which one nucleophile is substituted for
another.
- General equation :
-
Nu
R
R C X
R
nucleophile
substitution
R
Nu C R
R
X-
-
Nu = nucleophile
X- = leaving group
- substitution take place on an sp3 hybridized carbon atom.
•
Examples:
i) haloalkanes / alkyl halides undergo nucleophilic substitution
with hot aqueous sodium hydroxide to form alcohols and halide
ions.
- hydroxide ion (OH-) acts as nucleophile
- heterolytic fission of the C-Cl covalent bonds occurs.
CH3CH2Cl
OH
ethyl chloride
reflux
CH3CH2OH
ethanol
Cl-
CH3CH2OH
alcohol
X
OH- (hydroxide ion) = nucleophile
or
CH3CH2X
-
OH
reflux
Alkyl halides
OH- (hydroxide ion) = nucleophile
-
ii) reaction of alkyl halides with cyanide ion
-
R-X
alkyl halide
C N
R-C N
cyanide (nucleophile)
-
X
nitrile
Example:
CH3CH2CH2I
1-iodopropane
-
C N
cyanide (nucleophile)
CH3CH2CH2CH2CN
butanenitrile
I-
ELIMINATION REACTIONS
• A small molecule is removed from a larger molecule to produce
a double bond or a triple bond.
• Reverse of addition reactions.
• Examples:
i) dehydration (removal of H2O) of alcohol to form alkene.
C C
H OH
alcohol
C C
H2O
alkene
Example:
H H
H C C H
H OH
ethanol
H H
H C C H
ethene
H2O
ii) dehydrohalogenation (removal of hydrogen and a halogen
atom) of alkyl halide to form alkene.
C C
H X
alkyl halide
C C
HX
alkene
HX = HCl or HBr or Hl
Example:
H H
CH3 C C H
H Cl
chloropropane
H H
CH3 C C H
propene
HCl