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Chemistry
Alkyl and Aryl halides–1
Session objectives
1. Classification
2. Nature of C — X bond
3. Preparation of alkyl halides
4. Physical and chemical properties of alkyl halides
5. Substitution reaction: SN1 and SN2 reactions
Classification
Alkyl and aryl halides
MonohalogenDihalogen
10 20 30 vicinal Geminal
H
H
X
C C C H
X
H X HR
R
R XC
H
H
R XC
R
H
R XC
H
H
X
C C C H
H
H X H
Nextslide
Classification
Alkyl and aryl halides
Trihalogen PolyhalogenExamples1. CHCl32. CHI3
Examples1. CCl42. PFC3. p–dichlorobenzene4. DDT5. BHC6. Westron7. Westrosol
Unsaturated
Examples1. Vinyl halide
H2C=CHX2. Allyl halide
CH2x–CH=CH2
Previousslide
Nature of C–X Bond
C : X C — X+ –
or+ –
- -Nu +R - X R - Nu+ X :
Preparation of Alkyl Halides
By direct halogenation
4 2 3 2 2 3 4Mixture of products
CH +Cl CH Cl+CH Cl +CHCl + CCl U V
light
The substitution of H-atom in halides follows the order:
tert-hydrogen > sec-hydrogen > p-hydrogen
To get maximum yield of mono halogenated compound we should take alkene in excess.
Preparation of Alkyl Halides
By addition of hydrogen halides on alkenes
CH3 — CH CH2 + HBr CH3 — CH — CH3
Br
CH3 — CH CH2 ROORCH3 — CH — CH2Br CH3 — CH2 — CH2Br
H
Propene
HBr
sec-free radicalsstable
Preparation of Alkyl Halides
By alcohol
(a) Reaction with halogen acid
2ROH HX RX H O
(b) Reaction with phosphorous halides
3 3 3
5 3
3ROH PX 3RX H PO
ROH PX RX PX HX
(c) Reaction with thioxyl chloride
2 2ROH SOCl RCl SO HCl
Ask yourself
Which is the best method for preparation of alkyl chlorides?
Thioxyl chloride method because product obtained aregaseous in nature
Preparation of Alkyl Halides
By Finkelstein reaction(Halide exchange reaction)
3 2 3 2CH CH Cl NaI CH CH I NaCl
3 2 3 22CH Cl HgF 2CH F HgCl
From ethers
heat
2Excess
R O R 2HX 2RX H O
Preparation of Alkyl Halides
Borodine-Hunsdiecker reaction
4CCl2 2 2RCO Ag Br RBr CO AgBr
4CCl2 2RCOOAg Cl RCl CO AgCl
The yield of halide decreases in the order: 1° > 2° > 3°
Yield of alkyl chloride is less than alkyl bromide.Mechanism is through free radical formation.
Iodides cannot be prepared by this method since iodine forms esters instead of alkyl halide (Birnbaum-Simonini reaction)
2 2 2 22RCO Ag I RCO R CO 2AgI
Preparation of Alkyl Halides
Preparation of allylic and benzylic halides
CH3
X2
CH2X
HXh
CH3 — CH CH2CH2 — CH
Cl
CH2
Cl2 / h or
At 400° Cat low conc.
H2C CH — CH2 — CH3 H2C
Br
CH — CH — CH3NBS
Preparation of Alkyl Halides
Preparation of vinyl halides
C H 2C H 2 H 2C C H — C l
C H 3C O C l / a lu m in a
H C l
V in y l ch lo rid e
Physical and Chemical Properties of Alkyl HalidesPhysical properties
1. Alkyl halides are insoluble in water but soluble in organic solvents.
2. Decreasing order of their densities is:Iodide > Bromide > Chloride > Fluoride
3. Decreasing order of their boiling points for the same alkyl or aryl group is:Iodide > Bromide > Chloride > Fluoride
4. Their stability order is:RF > RCl > RBr > RISince C — X bond strength decreases in the same order.
5. Alkyl halides are insoluble in water although it has polar nature.
Chemical properties
(i) Reduction
2 5Na C H OH3 2 2 6CH CH Br 2[H] C H HBr
Red P
3 7 3 8 2150 C
C H I HI C H I
(ii) Reaction with metals
dry
ether2R — X 2Na R — R NaI
22R — X 2Na R — R ZnX
(iii) Friedel-Crafts’ reaction
3anhyd. AlCl3 6 6 6 5 3CH Br C H C H CH HBr
Chemical properties
(iv) Reaction with alc. KOH
alc KOH3 2 2 3 2CH CH CH Br CH CH CH HBr
Br
alc KOHCH3 — CH — CH2 — CH3
2-Bromobutane CH — CH2 — CH3CH3 – CH CH – CH3 + CH2
2-Butene (Major)
1-Butene (Minor)
Chemical properties
Reaction with aq. KOH
RX KOH(aq.) KOH KX
Reaction with moist Ag2O
2 22RX Ag O H O 2R – OH 2AgX
Reaction with dry Ag2O
22RX Ag O R – O – R 2AgX
Williamson synthesis
Rx RONa R – O – R NaX
Rx R`ONa R – O – R` NaX
Chemical properties
RX KCN RCN KX
Alkyl cyanide
H / OH2 3
(a) complete hydrolysis
RCN 2H O RCOOH NH
2 2H O2 2
(b) Partial hydrolysis
RCN H O RCONH
Na–alcohol2 2
(c) Reduction
RCN RCH NH
Reaction with alkyl cyanide
Chemical properties
Reaction with amines
3 2 3 2CH Br KNH CH NH KBr
Reaction with silver nitrite:
2 5 2 2 52C H Br 2AgNO C H ONO 2AgBr
Chemical properties
Alkyl halides react with sodium-lead alloy to form tetra-ethyl lead
2 5 2 5 4Sodium lead Tetra ethyl
alloy lead TEL
4C H Cl 4Pb Na C H Pb 4NaCl 3Pb
Reaction with magnesium:
dry
ether Alkylmagnesiumhalide
Alkyl lithium
RMgX
alkylhalide
Li
ether
RX Mg dry
R — X R — Li LiX
Nucleophilic aliphatic substitution
The SN1 reaction mechanism:
Ist step
C
CH3CH3
CH3
IInd step
(Racemic mixture)
C Cl
CH3
CH3
CH3 slow
C
CH3CH3
CH3
fast
OH
C OH
CH3
CH3
CH3
CH3
HO C CH3
CH3
+
Factors effecting substitution
1. Structure of RX
• Reactivity for SN1 3° > 2° > 1°.
• Reactivity for SN2 1° > 2° > 3°.
2. Solvent
•Polar protic solvents (e.g., H2O,Alcohol etc) SN1
•non-polar (CCl4 , CHCl3) or polar aprotic (DMSO,DMF) solvents SN2 reactions.
Factors effecting substitution
3. Nature of Nucleophile Low concentration favour SN1 reactions while high conc. FavourSN2 reactions.
Basicity is the tendency toabstract H+ and nucleophilicity isthe tendency to attack on carbon.
4. Effect of leaving group same for SN1 and SN2 reaction.R – I > R – Br > R – Cl > R – F
Substitution versus Elimination
SN2 vs. E2
Effect of substrate
For SN2, the reactivity order is 1° > 2° > 3°
For E2, the reactivity order is 3° > 2° > 1°
Thank you