Halogen compounds 2

1. ORGANIC CHEMISTRY
UNIT – III
Halogen compounds
By
Anusha Karumuri,
Lecturer in chemistry,
GCW(A),Guntur.

2. • 1. Halogen compounds 5 h
• Nomenclature and classification of alkyl (into primary, secondary, tertiary), aryl,
aryl alkyl, allyl, vinyl, benzyl halides. Nucleophilic aliphatic substitution reaction-
classification into SN1 and SN2 – reaction mechanism with examples – Ethyl
chloride, t-butyl chloride and optically active alkyl halide 2-bromobutane.
• Additional Inputs: Factors affecting substition reactions, SNi, neighbouring
group participation reactions.

3. Nomenclature of alkylhalides:
IUPAC Nomenclature(Haloalkane):
1.Selection of longest chain,
2.Naming the halogen as substituent,
3.If more than one halogen present,order names alphabetically,
4.If more than one similar halogens use di,tri….etc before the name of the halogen,
EX:
CH3F-Fluoromethane CH3CH2CH2F-Fluoropropane
CH3Cl-Chloromethane
CH3Br-Bromomethane
CH3I-Iodomethane
CH3CH2F-Fluoroethane CH3CH2CH2CH2F-Fluorobutane
CH3CH2Cl-Chloroethane
CH3CH2Br-Bromoethane
CH3CH2I-Iodoethane
2-Floropropane
2-Florobutane
2-Fluoro-2-methylpropane

4. • Common name: (Alkylhalide)
• Ex:
CH3F-Methylfluoride CH3CH2CH2F-Propylfluoride
CH3Cl-Methylchloride
CH3Br-Methylbromide
CH3I-Methyliodide
CH3CH2F-Ethylfluoride CH3CH2CH2CH2F-Butylfluoride
CH3CH2Cl-Ethylchloride
CH3CH2Br-Ethylbromide
CH3CH2I-Ethyliodide

5. • The terms n-,Iso,Neo
• n- normal without branching
• EX:
n- Propylchloride
Isopropylchloride
n-Pentylbromide
Isopentylchloride

6. • Classification of alkylhalides
into
primary,secondary, tertiary,
aryl, aralkyl, allyl, vinyl, benzyl
halides.

7. •

8. • Primary,secondary,tertiary alkylhalides:
• When the halogen is attached to the carbon which is bonded with other carbon-10
• When the halogen is attached to the carbon which is bonded with 2 other carbons-20
• When the halogen is attached to the carbon which is bonded with 3 other carbons-30
Ex:
CH3CH2F-Fluoroethane
CH3CH2Cl-Chloroethane 10 20
CH3CH2Br-Bromoethane
CH3CH2I-Iodoethane
CH3CH2CH2CH2F-Fluorobutane
30

9. • aryl halides(Haloarenes):

10. •Aralkylhalides:
• Aralkyl halides are the group of organic compounds in which halogen atom
is linked to the carbon atom of the side chain which is attached to the
carbon atom of benzene ring.
Benzalchloride

11. • Vinyl ,Allyl halides:
In organic chemistry, a vinyl halide is a compound with the formula
CH2=CHX (X = halide). The term vinyl is often used to describe any alkenyl
group. For this reason, alkenyl halides with the formula RCH=CHX are
sometimes called vinyl halides.
X is bonded to sp2 carbon of alkene.
CH2=CHX
An allyl group is a substituent with the structural formula H2C=CH−CH2R,
where R is the rest of the molecule. It consists of a methylene
bridge (−CH2−) attached to a vinyl group (−CH=CH2).
Allylchloride

12. • R-X are highly reactive.
• The order of reactivity of alkylhalides are RI>RBr>RCl>RF
• If the halide is same 30>20>10
• R+-X- is polar
Chemical properties:
They undergo 1)Displacement reactions(or substitution reactions)
2)Elimination reactions
3)reduction reactions
4)with metals.
Nucleophilic substitution reactions:
R-X + Y- R-Y+X-

14. SN1 SN2
1.SN1 means Nucleophilic substitution
unimolecular
The first step is the slow and rate
determining step.
i,.e in the rate determining step only one
molecule is involved.
2.It is stepwise mechanism
3.Rate of reaction depends on the
concentration of the substrate only
Rα[A]
The reaction mechanism follows first order
kinetics in polar protic solvents .
Ex:.
H2O,NH3,CH3COOH,C2H5OH,CH3OH,propanol,
Tertiarybutanol etc.
Weak nucleophile
1. SN2 means Nucleophilic substitution
bimolecular
2.It is concerted mechanism
3.Rate of reaction depends on the concentration
of both reactant(Nu- )and substrate(R-X).
Rα[A][B]
Reaction mechanism follows second order
kinetics in polar aprotic solvents.
Ex:
Strong nucleophile

15. •
4.Older bond is broken first and then only
new bond is formed.
5.The substitution is from either front side or
from back side.
6.SN1 reactions follow carboniumion
mechanism as the intermediate formed is
carbonium ion.
7.Order of reactivity of alkylhalides are as
follows
tButylhalide>Benzyl>Allyl>30>20>10>CH3
+>viny
l>arylhalide.
4.The older bond is broken and
simultaneously new bond is formed.
5.The substitution is always from back side.
6.SN2 reaction involves formation of complex
as an intermediate which contains both
reactant and substrate.
7. Order of reactivity of alkylhalides are as
follows
vinyl<Arylhalide<t-
Butylhalide<Benzyl<Allyl<30<20<10<CH3
+

16. 8.It is accompanied with 50% retention and
50% retention of configuration.Thereby SN1
substitution made at asymmetric carbon
atom leads to recemisation.
Reaction mechanism:
8.It is accompanied by 100% inversion of
Configuration called walden inversion.
Reaction mechanism:
Ex:Ethylchloride(a 10 alkylhalide)

17. • EX: optically active alkyl halide 2-bromobutane:
• 20 Alkyl halides can folloe nucleophilic substitution either by SN1 or SN2.
• It has a chiral carbon.It can undergo both SN1 and SN2 depending on conditions.
• SN1 mechanism SN2 mechanism
•

18. 9.The carbocations are stabilised by
aromatic >resonance> hyperconjugation> Inductive
effect.
In Benzylhalides and allylhalides the carbocation is
stabilised by resonance.so more
reactive than R-X
In Allylhalides the carbocation is stabilised by resonance:

19. Hyperconjugation

20. •In vinyl and arylhalides due to partial double bond character between
halogen and unsaturated carbon they are inert to nucleophilic
substitution.
In polar protic solvents benzyl and
Allyl carbocations are formed at room temperature
whereas primary carbocations are formed by boiling,
Vinyl at 2000c
Aryl carbocations are difficult to form.

21. • 10.Stable the carbocation the rate
of solvolysis is more.

22. Factors effecting Nucleophilic substitution Reactions

23. • 1.Nature of the substrate:
• For SN of R-F
• R-Cl
• R-Br
• R-I
• The rate of hydrolysis of R-X in polar solvents is as follows:
R-I>R-Br>R-Cl>R-F as in R-I the bond length is more ,more easy to cleave.So the
reactivity of R-X towards SN in both SN1 and SN2 is R-I>R-Br>R-Cl>R-F .
• Order of reactivity of alkylhalides for SN1are as follows where polar effects(I-
effect,M-effect,R-effect,Hyperconjugation etc.) stabilise the carbocation
t-Butylhalide>Benzyl>Allyl>30>20>10>CH3
+>vinyl>arylhalide.
• Order of reactivity of alkylhalides for SN2 are as follows :
• t-Butylhalide<Benzyl<Allyl<30<20<10<CH3
+<vinyl<arylhalide.
• Steric restriction opposes SN2.

24. • 2. Nature of Leaving group:
• The rate of SN1 reaction is largely influenced by the nature of the leaving
group.
• The rate of ionisation is affected by the stability of the leaving group,X-.The
more stable the LG,the more easily it will lost,favour SN1 reactions.
• Less basic groups are better LG (EX: Basicity order :I-<Br-<Cl-<F-,The conjugate base of
strong acids)because a strong base(conjiugate base of weakacid ,Ex:OH-,OR-,R2N- etc)
has a greater tendency for a backward direction in the reversible reaction.
HA H++A-
• Good leaving group after removing form neutral species.
EX:
• >
OH- H2O(neutral species)
• Larger –ve ions are GLG.
F->Cl->Br->I-
.

25. • When the E.N of the central atom is more then that will be GLG
• EX:
CH3
- < NH2
- < OH-
• Cl- >OH-(larger size)
• N2(g)>H2O(l)
• The decreasing order of leaving group tendency of some groups is as follows:..
>I-> Br->CF3COO->OH->Cl->F->CH3COO-
> > >
>

26. • 3. Nucleophile nature:
• Nucleophilicity refers to the ability of a nucleophile to displace a leaving group in
a substitution reaction. There are various trends in nucleophilicity .
• There is a trend within a period, a trend within a group, a trend based on the
charge of the nucleophile, and a steric effect of the nucleophile.
• Nucleophiles are also bases, and they can abstract protons in elimination
reactions. However, although nucleophilicity and basicity are related to the
availability of the same electron pair, the reactions of a series of nucleophiles do
not necessarily parallel those of the same species as bases.
• Within a period, nucleophilicity parallels basicity and decreases from left to right
in the periodic table for elements in similarly structured species with the same
charge. For example, hydroxide ion is a better nucleophile than fluoride ion.
Nucleophilicity is decreased by hydrogen bonding of protic solvents such as
alcohols. CH3
->NH2
->OH-,in aprotic solvents the order is reverse.
• Within a group, the order of nucleophilicity is opposite to the order of basicity.
This order of nucleophilicity is related to the polarizability of the nucleophile.
The order I− > Br− > Cl− is one that we encounter many times in the study of
− −

27. • Charge has a large effect on nucleophilicity. A species with a negative charge is more
nucleophilic than a neutral species with a similar structure. For example, alkoxide ion,
RO−, is a better nucleophile than ROH.
• A nucleophile must approach a carbon reaction center to form a bond.
Therefore, steric hindrance affects the rate of reaction. Sterically hindered nucleophiles
react at a slower rate than similarly charged, smaller nucleophiles containing the same
nucleophilic element. For example, tert-butoxide reacts more slowly than ethoxide in
SN2 reactions
• Strong nucleophiles favour SN2 mechanism while Weak Nucleophiles favour SN1
mechanism.
• Strength of Nucleophile is as follows:
PhS- >CN->I->EtO->OH->Br->PhO->Cl->Me3N
• 4. Concentration of the Nucleophile:
• Concentration of Nucleophile effects SN2.
• Concentration of Nucleophile doesn’t effects SN1as rate depends only on substrate
concentration.

28. • 5.Nature of solvent:
• polar protic solvents favour SN1 .
• Ex:. H2O,NH3,CH3COOH,C2H5OH,CH3OH,propanol,t-butanol etc
• More polar the solvent more nucleophilic it will be.
• polar aprotic solvents favour SN2.
• Ex:THF,acetone,DMSO etc..

29. • SN i: (Substitution nucleophilic internal)
In this process part of leaving group which attacks the substrate detaches itself from the rest of the leaving group
• EX: conversion of (R)-2-butanol to (R)-2-cholrobutane with SOCl2 in non-polar solvent and absence of base.
• The product formed is with complete retention of configuration,i.e.,in which starting material and product have
the same configuration.
• The mechanism appears to involve the formation of intermediate chlorosulphite ester,ROSOCl(R+sec-
butylgroup),which dissociates into an intimate ion pair,R+:-OSOCl as in SN1 mechanism.
• The Cl with pair of electrons of the anion attacks the R+ from the same side of the carbocation from which–OSOCl
departed and product(RCl) is formed with complete retention of configuration.

30. • It is interesting that if a tertiary amine such as pyridine is added to the reaction
mixture,the product RCl is found now to have undergone inversion of
configuration.
• The pyridine coordinates with HCl,produced during the formation of intermediate
Clorosulphite from ROH an SOCl2,to form pyridinehydrochloride and the Cl- is an
effective nucleophile.
The displacement of the chlorosulphinateester by Cl- via SN2 mechanism gives
product with complete inversion of configuration.

31. •

32. • NGP(neighbouring group participation):
When a molecule capable of undergoing SN reaction has a nucleophilic
group(e.g.Z) present in the proper position for backside attack i.e. attack anti to
the leaving group(X) both the kinetics and stereochemistry of SN reactions are
strongly affected.
The involvement of the near by nucleophilic substituent in an SN process is termed
NGP.

34. • In the first step the neighbouring group attack carbon at the vtreaction
centre(SN2 attack)and the leaving group is lost to give a bridged intermediate.
• This is then attacked in the second step by an external nucleophile(Y:) and the
interna nucleophile goes back to where it came from, the net result is two
consecutive SN2 reactions leading to retention of configuration at the reaction
carbon.
• This all happens when molecular geometry required for participation (SN2
conditions) can be achieved.
• It is favoured in the first step ,3 or 5 membered ring is formed and least
favourable when it forms 4-membered ring.
• Participation group with a lone pair,pi electron of a doubler bond or even sigma
electron of a single bond.