Halogen derivates of alkanes HDA

Halogen derivatives
PRESENTED BY:
FREYA CARDOZO
PRESENTED BY: FREYA CARDOZO 1

Classification based on Hydrocarbon
skeleton
On the basis of hydrocarbon skeleton to which halogen atom is bonded, the halogen derivatives
are classified as haloalkanes, haloalkenes, haloalkynes and haloarenes.

Based on no. Of halogen atoms
On the basis of number of halogen atoms, halogen derivatives are classified as mono, di, tri or
poly halogen compounds.

Monohalogen compunds
Alkyl halides or haloalkanes
Allylic halides
Benzylic halides
vinylic halides
Haloalkynes
Aryl halides or haloarenes

Haloalkanes or alkyl halides
halogen atom is bonded to sp3 hybridized carbon which is a part of saturated carbon skeleton.
Alkyl halides may be primary, secondary or tertiary depending on the substitution state of the
carbon to which halogen is attached

Identify

Allylic halides
In allylic halides, halogen
atom is bonded to a sp3
hybridized carbon
atom next to a carbon-carbon double bond

Benzylic halides
In benzylic halides halogen atom is bonded to a sp3 hybridized carbon atom which is further
bonded to an
aromatic ring.

Vinylic halides
In vinylic halides halogen
atom is bonded to a sp2 hybridized carbon atom of aliphatic chain.
Vinylic halide is a haloalkene.

Haloalkynes
When a halogen atom is
bonded to a sp hybridized carbon atom it is
a haloalkyne.

Haloarenes or aryl halides
In aryl
halides, halogen atom is directly bonded to
the sp2 hybridized carbon atom of aromatic
Ring.

Things you need to know!
Carbon ke 4 haath hai, so it can always form only 4 bonds
-ane = single bonds, ene = double bond, yne = triple bond, -ol= OH group i.e. alcohol
Rememeber
1 C Methane F Fluoride Fluoro
2 C Ethane Br Bromine Bromo
3 C Propane Cl Chlorine Chloro
4 C Butane I Iodine Iodo
5 C Pentane
6 C Hexane

Preperation of alkyl
halide

Four main methods
• Using hydrogen halides (HX)
• Using phosphorus halide (PCl3 ; PCl5)
• Using thionyl chloride (SoCl2)
From alcohol
• Alkane
• Alkenes – For asymmetrical alkenes  Markonikoff and Anti-Markonikoff
From hydrocarbon –
alkene
• Finkelstein reaction
• Swartz reactionHalogen exchange
Electrophilic
substitution

From Alcohols- Hydrogen acids
R-OH + HX

From HCl

Using HBr

Using HI

Phosphorus trichloride PCl3

Phosphorus pentachloride PCl5

Thionyl Chloride SOCl2

From Hydrocarbons
Alkanes
Alkenes
Alkenes + CCl4

Write reaction and iupac name of major
product

Halogen exchange reaction
Finkelstein reaction

Swartz reaction

Electrophilic substitution

Questions
Convert
1. But-1-ene to n-butyl iodide
2. Aniline to chlorobenzene

Physical properties

Bond strength
Carbon atom that carries halogen develops a partial positive charge while the halogen carries a
partial negative charge. Thus carbon-halogen bond in alkyl halide is a polar covalent bond.
Therefore alkyl halides are moderately polar compounds
Size of the halogen atom increases from fluorine to iodine. Hence the C-X bond length increases.
The C-X bond strength decreases with an increase in size of halogen.
This is because as the size of p-orbital of halogen increases the p-orbital becomes more
diffusedand the extent of overlap with orbital of carbon decreases

Boiling point
Boiling points of alkyl halides are considerably higher than those of corresponding alkanes due
to higher polarity and higher molecular mass
1. For alkyl halides with same R group the B.P depends upon the size of halogen atom.More
larger the atom more is B.P . Because Vander waals forces increase
Thus boiling point of alkyl halide decreases in the order RI > RBr > RCl > RF

2. With same X atom and different R group...
Longer R group chain or more number of carbon atoms
More is the Vander waal forces
Higher B.p

3. For isomeric alkanes i.e R group with same number of carbon atoms
More the branching
Lesser the surface area
Lesser Vander waal forces
Thus lesser B.P

Question
Arrange the following
compounds in order of increasing boiling
points : bromoform, chloromethane,
dibromomethane, bromomethane.

Solubility
Though alkyl halides
are moderately polar, they are insoluble in
water. It is due to inability of alkyl halides to
form hydrogen bonds with water. Attraction
between alkyl halide molecules is stronger
than attraction between alkyl halide and
water. Alkyl halides are soluble in non-polar
organic solvents.
Aryl halides are also insoluble in water
but soluble in organic solvents.

Alkyl halides though polar are immiscible with water.Give reason

Reactions of alkyl
halides
• L A B T E S T
• S U B S T I T U T I O N R E A C T I O N S
• D E H Y D R O H A L O G R N A T I O N / B E T A E L I M I N A T I O N
•

Lab test

Substitution reactions
Formation of
1. alcohol
2. Amines
3. cyanide
4. Isocyanide
5. Esters
6. Ethers
7. Nitroalkanes
8. Alkyl nitrite

Optical activity

Draw 2-chlorobutane

Carbon atom in a molecule which carries four different groups/atoms is called chiral carbon
atom.
Thus, the C-2 in 2-chlorobutane is a chiral carbon. Chiral atom in a molecule is marked with
asterisk (*).

Optical isomers and optical activity
The stereoisomerism in which the isomers have different spatial arrangements of groups/atoms
around a chiral atom is called optical isomerism.
The optical isomers differ from each other in terms of a measurable property called optical
activity

Plane polarized light
light having oscillations only in one plane perpendicular to direction of propagation of light is
known as plane polarized light.

Definitions
property of a substance by which it rotates plane of polarization of incident plane polarized light is
known as optical activity.
The compounds which rotate the plane of plane polarized light are called optically active compounds
and those which do not rotate it are optically inactive compounds.
Optical activity of a substance is expressed numerically in terms of optical rotation. The angle
through which a substance rotates the plane of plane polarized light on passing through it is called
optical rotation.
In accordance with the direction of optical rotation an optically active substance is either
dextrorotatory or laevorotatory.
A compound which rotates the plane of plane polarized light towards right is called dextrorotatoryand
designated by symbol d- or by (+)sign.
A compound which rotates plane of Plane polarized light towards left is called laevorotatory and
designated by symbol l-or by (-) sign.

Enantiomers
The optical isomers which are non-superimposable mirror image of each other are called
enantiomers or enantiomorphs or optical antipodes.
Enantiomers have equal and opposite optical rotation. Thus, enantiomers are a kind of optical
isomers.
Enantiomers have identical physical properties (Such as melting point, boiling points, densities,
refractive index) except the sign of optical rotation.
The magnitude of their optical rotation is equal but the sign of optical rotation is opposite.
They have identical chemical properties except towards optically active reagent.

Racemic mixture
An equimolar mixture of enantiomers (dextrorotatory and laevorotatory) is called racemic
modification or racemic mixture.
A racemic modification is optically inactive because optical rotation due to molecules of one
enatiomer is cancelled by equal and opposite optical rotation due to molecules of the other
enantiomer.
A racemic modification is designated as (dl) or by (±) sign.

SN mechanism

Distinguish between

Factors influencing SN1 and SN2
mechanism
Nature of substrate
Nucleophilicity of reagent
solvent polarity

Nature of substrate
1° alkyl halide – SN2 reaction
3° alkyl halide – SN1 reaction
T.S in SN2 is pentacoordinate carbon which is crowded this not favoured by 3°
Steric hinderance doesn’t allow 3° to proceed by this mechanism
For SN1 it proceeds with formation of carbocation which is stabilized by hyperconjugation and +I
inductive effect of alkyl group
Thus favours 3° R-X

Nucleophilicity of reagent
Nucleophilicity is similiar to concept of Lewis base
Lewis base- donates an electron pair
easier the nucleophile donating the electron pair better Nucleophilic reagent
In SN2 reaction rate depends on nucleophile thus stronger nucleophile will be needed for such
reaction
Poor nucleophile undergo SN1 beacuse rate is independent of nucleophile

Solvent polarity
SN1 reactions proceed rapidly in protic and polar solvents
SN1mechanism proceeds via formation of carbocation intermediate.
A good ionizing solvent, polar solvent, stabilizes the ions by solvation.
Cations are poorly solvated but anions solvation is of key importance
Anions are solvated by hydrogen bonding solvents, that is, protic solvents

aprotic solvents or solvents of low polarity will favour SN2 mechanism.
Polar protic solvents usually decrease the rate of SN2 reaction.
In the rate determining step of SN2 mechanism substrate as well as nucleophile is involved.
A polar solvent stabilizes nucleophile (one of the reactant) by solvation. Thus solvent deactivates
the nucleophile by stabilizing it.

Which of the following
two compounds would react faster by SN2
mechanism and Why ?
1-chlorobutane and 2-chlorobutane

Primary allylic and primary benzylic halides show higher reactivity by SN1 mechanism than other
primary alkyl halides. Explain.

Haloarenes

Sandmeyers reaction
Aniline reacts with nitrous acid HNO2 (NaNo2 + HCl) to give benzene diazonium chloride which
when treated with copper chloride forms Chlorobenzene and nitrogen gas.

Reaction with metals

Fittig Reaction

Wurtz fittig Reaction

G.R
Why cleavage of C-X bonds in haloarenes proceeds with great difficulty?
Resonance effect
Sp2 hybridization of carbon chlorine

How to break C-X bond
Placing –I or electronwithdrawing groups at the o or p position
More grps easier reaction

Nucleophilic substitution

GR. Therefore Cl is o-/p- directing but
ring deactivating group.

GR.
Aryl halides undergo electrophilic substitution reaction slowly as compared to benzene.
In resonance structures of chlorobenzene elelctron density is relatively more at ortho and para
position.
Therefore incoming electrophilic group is more likely to attack at these positions. But due to
steric hinderance at ortho position, para product usually predominates.
In haloarenes, halogen atom has strong electron withdrawing inductive effect (-I).
This deactivates the ring and electrophilic substitution reaction occurs slowly.

Halogenation

Nitration

SULphonation

Fridel crafts Alkylation

Fridel crafts acylation

Halogen derivates of alkanes HDA

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