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notes contain organohalogen a.k.a alkyl hallide
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ORGANOHALOGEN
DR. ROSWANIRA ABDUL WAHAB
DEPARTMENT OF CHEMISTRYFACULTY OF SCIENCE
UNIVERSITI TEKNOLOGI MALAYSIAC18-311
Classes of HalidesClasses of Halides
Alkyl: Halogen, X, is directly bonded to sp3 carbon.
Vinyl: X is bonded to sp2 carbon of alkene.
Aryl: X is bonded to sp2 carbon on benzene ring.
Examples:
C
H
H
H
C
H
H
Br
alkyl halide
C CH
H
H
Cl
vinyl halide
I
aryl halide
2
Polarity and ReactivityPolarity and Reactivity
Halogens are more electronegative than C.
Carbon-halogen bond is polar, so carbon has partial positive charge.
Carbon can be attacked by a nucleophile.
Halogen can leave with the electron pair.
3
Classes of Alkyl HalidesClasses of Alkyl Halides
Methyl halides: only one C, CH3X
Primary: C to which X is bonded has only one C-
C bond.
Secondary: C to which X is bonded has two C-C
bonds.
Tertiary: C to which X is bonded has three C-C
bonds.
4
DihalidesDihalides
Geminal dihalide: two halogen atoms are bonded
to the same carbon
Vicinal dihalide: two halogen atoms are bonded
to adjacent carbons.
C
H
H
H
C
H
Br
Br
geminal dihalide
C
H
H
Br
C
H
H
Br
vicinal dihalide
5
IUPAC NomenclatureIUPAC Nomenclature
Name as haloalkane.
Choose the longest carbon chain, even if the
halogen is not bonded to any of those C’s.
Use lowest possible numbers for position.
CH3 CH CH2CH3
Cl CH3(CH2)2CH(CH2)2CH3
CH2CH2Br
2-chlorobutane 4-(2-bromoethyl)heptane
6
““Trivial” NamesTrivial” Names
CH2X2 called methylene halide.
CHX3 is a haloform.
CX4 is carbon tetrahalide.
Examples:
CH2Cl2 is methylene chloride
CHCl3 is chloroform
CCl4 is carbon tetrachloride.
7
Uses of Alkyl HalidesUses of Alkyl Halides
Solvents - degreasers and dry cleaning fluid
Reagents for synthesis of other compounds
Anesthetic: Halothane is CF3CHClBr
CHCl3 used originally (toxic and carcinogenic)
Freons, chlorofluorocarbons or CFC’s
Freon 12, CF2Cl2, now replaced with Freon 22,
CF2CHCl, not as harmful to ozone layer
Pesticides - DDT
8
PHYSICAL PHYSICAL PROPERTIESPROPERTIES
9
Dipole MomentsDipole Moments
= 4.8 x x d, where is the charge (proportional
to EN) and d is the distance (bond length) in
Angstroms.
Electronegativities: F > Cl > Br > I
Bond lengths: C-F < C-Cl < C-Br < C-I
Bond dipoles: C-Cl > C-F > C-Br > C-I
1.56 D 1.51 D 1.48 D 1.29 D
10
Boiling PointsBoiling Points
Greater intermolecular forces, higher b.p.
dipole-dipole attractions not significantly
dissimilar for different halides
London forces greater for larger atoms
Greater mass, higher b.p.
Spherical shape decreases b.pt. (due to branching that
lower surface area that reduces intermolecular interaction)
Example:
(CH3)3CBr CH3(CH2)3Br
73C 102C
11
12
DensitiesDensities
Alkyl fluorides and chlorides less dense than
water.
Alkyl dichlorides, bromides, and iodides more
dense than water.
13
Preparation of Alkyl HalidesPreparation of Alkyl Halides
Free radical halogenation of alkanes
produces mixtures, not good lab synthesis
unless: all H’s are equivalent, or
halogenation is highly selective.
Free radical allylic halogenation
produces alkyl halide with double bond on
the neighboring carbon.
14
Halogenation of AlkanesHalogenation of Alkanes All H’s equivalent. Restrict amount of halogen to
prevent di- or trihalide formation
Highly selective: bromination of 3carbon
+ HBr
H
BrhBr2+
H
H
90%
+ HBrCH3 C
CH3
CH3
Brh
Br2+CH3 C
CH3
CH3
H
15
Allylic HalogenationAllylic Halogenation
Allylic radical is resonance stabilized.
Bromination occurs with good yield at the allylic position (sp3 carbon next to C=C).
Avoid a large excess of Br2 by using
N-bromosuccinimide (NBS) to generate Br2 as
product.
16
Free radical allylic halogenationFree radical allylic halogenation
17
18
Reaction MechanismReaction Mechanism
Free radical chain reaction– initiation, propagation, termination
H H
BrH
+ HBr
BrBr
H Br
+ Br
2 BrBr2h
19
Preparation of Alkyl Halides (2)Preparation of Alkyl Halides (2)
b)b) Halogenation of Alkene (Addition of HX)Halogenation of Alkene (Addition of HX)
20
Preparation of Alkyl Halides (3)Preparation of Alkyl Halides (3)
c) Halogenation of Alkene (Addition of Halogens)c) Halogenation of Alkene (Addition of Halogens)
21
Preparation of Alkyl Halides (4)Preparation of Alkyl Halides (4)
d) RX from ROH (alcohol)d) RX from ROH (alcohol)
(Reaction with HX)(Reaction with HX)
22
(Reaction with HCl – Lucas Test)(Reaction with HCl – Lucas Test)
(Reaction with PBr(Reaction with PBr33))
23
(Reaction with SOCl(Reaction with SOCl22))
24
REACTIONS OFREACTIONS OF ALKYL HALIDESALKYL HALIDES
25
Nucleophilic Substitution & Nucleophilic Substitution & Elimination ReactionsElimination Reactions
26
Nucleophilic Substitution ReactionsNucleophilic Substitution Reactions
The halogen atom on the alkyl halide is replaced with
another group.
Since the halogen is more electronegative than carbon, the
C-X bond breaks heterolytically and X- leaves; halide is a
good leaving group (conjugate base for strong acid).
The group replacing X- is a nucleophile.
C C
H X
+ Nuc:-C C
H Nuc
+ X:-
27
What are Nucleophiles? Species rich in electrons
(in the form of -ve charge, lone pair of electrons, or )
Eg: a) Strong Nu-: NaOH, NaBr, NaOCH3, NaNH2, HCN
b) Weak Nu-: NH3, H2O, ROH, RNH2
The –vely charge Nu- in (a) have more electrons and therefore are stronger Nu- than neutral molecules in (b) with just lone pair of electrons, meaning (a) can react by donating its electrons much easier than (b).
Also I- > Br-> Cl-.
* Spesies in (a) can attack neutral R-X since it is a strong Nu- but
* Species in (b) prefers attacking a charged R-X in order to react since it is less powerful Nu- (since it itself is still a neutral molecule).
28
Second-Order Nucleophilic Substitution: Second-Order Nucleophilic Substitution: The SThe SNN2 Reaction2 Reaction
SN2 : substitution, nucleophilic, bimolecular
hydroxide ion is a strong nucleophile (donor of an
electron pair)
29
SSNN2 Mechanism2 Mechanism
Bimolecular Bimolecular : the transition state of the rate-limiting step
involves the collision of two molecules.
Concerted reactionConcerted reaction: new bond forming and old bond
breaking at same time.30
SSNN2: Reactivity of Substrate2: Reactivity of Substrate Carbon must be partially positive.
Must have a good leaving group
Carbon must not be sterically hindered.
31
Structure of SubstrateStructure of Substrate
Relative rates for SN2:
CH3X > 1° > 2°
Tertiary (3) halides do not react via the SN2 mechanism, due to steric hindrance.
32
Uses for SUses for SNN2 Reactions2 Reactions Synthesis of other classes of compounds. Halogen exchange reaction.
33
Nucleophilic StrengthNucleophilic Strength Steric EffectsSteric Effects Solvent EffectsSolvent Effects
Stronger nucleophiles react faster.
Strong bases are strong nucleophiles, but not all
strong nucleophiles are basic.
FACTORS AFFECTING SFACTORS AFFECTING SNN2 REACTIONS:2 REACTIONS:
34
35
A base is always a stronger nucleophile than its A base is always a stronger nucleophile than its conjugate acidconjugate acid
36
BasicityBasicity is defined by the equilibrium constant for abstracting a proton.
NucleophilicityNucleophilicity is defined by the rate of attack on an electrophilic carbon atom.
37
a) A species with a negative charge is a stronger
nucleophile than a similar neutral species. In particular, a
base is a stronger nucleophile than its conjugate acid.
OH- > H2O, NH2- > NH3
b) Nucleophilicity decreases from left to right in the
periodic table, following the increase in
electronegativity from left to right. The more electronegative
elements have more tightly held
nonbonding electrons that are less reactive toward
forming new bonds.
OH- > F-, NH3 > H2O
Trends in Nucleophilic StrengthTrends in Nucleophilic Strength
38
Trends in Nucleophilic StrengthTrends in Nucleophilic Strength
c) Nucleophilicity increases down the Periodic Table, following the increase in size and polarizability.
I- > Br- > Cl- > F-
39
Polarizability EffectPolarizability Effect
40
Bulky Nucleophiles: Steric EffectsBulky Nucleophiles: Steric Effects
Sterically hindered for attack on carbon, so weaker nucleophiles.
CH3 CH2 O ethoxide (unhindered)weaker base, but stronger nucleophile
C
CH3
H3C
CH3
O
t-butoxide (hindered)stronger base, but weaker nucleophile
41
Solvent Effects (1)Solvent Effects (1)
Polar protic solvents (O-H or N-H) reduce the strength of the nucleophile.
Hydrogen bonds must be broken before nucleophile can attack the carbon.
42
Solvent Effects (2)Solvent Effects (2)
Polar aprotic solvents (no O-H or N-H) do not form hydrogen bonds with nucleophile
Examples:
CH3 C Nacetonitrile C
O
H3C CH3
acetone
dimethylformamide (DMF)
CH
O
NCH3
CH3
43
Leaving Group AbilityLeaving Group Ability
Electron-withdrawing Stable once it has left Polarizable to stabilize the transition state.
44
SUMMARY OF SSUMMARY OF SNN2 REACTION2 REACTION
Substrate : CH3X > 1 RX > 2 RX (3 RX is not suitable)
Nucleophile: Strong nucleophile
Solvent: Less polar solvent
Rearrangement: Impossible (one step reaction)
45
Tertiary (3) halides do not react via the SN2 mechanism, due to steric hindrance.
46
Unimolecular nucleophilic substitution. Two step reaction with carbocation intermediate.
First-Order Nucleophilic Substitution: First-Order Nucleophilic Substitution: The SThe SNN1 Reaction1 Reaction
47
Reactivity of SReactivity of SNN1 Reactions1 Reactions
3° > 2° > 1° >> CH3X
Order follows stability of carbocations (opposite to SN2)
More stable ion requires less energy to form
48
Better leaving group, faster reaction (prefer SN2)
Polar protic solvent best: It solvates ions strongly with hydrogen bonding.
49
Rearrangements of CarbocationsRearrangements of Carbocations
Carbocations can rearrange to form a more stable
carbocation.
Hydride shift: H- on adjacent carbon bonds with C+.
Methyl shift: CH3- moves from adjacent carbon if
no H’s are available.
50
51
52
Hydride ShiftHydride Shift
CH3 C
Br
H
C
H
CH3
CH3CH3 C
H
C
H
CH3
CH3
CH3 C
H
C
H
CH3
CH3CH3 C
H
C
CH3
CH3
H
CH3 C
H
C
CH3
CH3
HNuc
CH3 C
H
C
CH3
CH3
H Nuc
53
Methyl ShiftMethyl Shift
CH3 C
Br
H
C
CH3
CH3
CH3CH3 C
H
C
CH3
CH3
CH3
CH3 C
H
C
CH3
CH3
CH3CH3 C
H
C
CH3
CH3
CH3
CH3 C
H
C
CH3
CH3
CH3
NucCH3 C
H
C
CH3
CH3
CH3 Nuc
54
SUMMARY OF SSUMMARY OF SNN1 REACTIONS1 REACTIONS
Substrate : benzyl > allyl > ~3 RX > 2 RX (1 RX and CH3X are unlikely)
Nucleophile: weak nucleophile
Solvent: polar protic solvent
Rearrangement: common to form most stable carbocation
55
Elimination ReactionsElimination Reactions
The alkyl halide loses halogen as a halide ion, and
also loses H+ on the adjacent carbon to a base.
A pi bond is formed. Product is alkene.
Also called dehydrohalogenation (-HX).
C C
H X
+ B:- + X:- + HB C C
56
Unimolecular elimination
Two groups lost (usually X- and H+)
Nucleophile acts as base (weak base)
Also have SN1 products (mixture)
First-Order Elimination: First-Order Elimination: The E1 ReactionThe E1 Reaction
57
58
E1 MechanismE1 Mechanism
Halide ion leaves, forming carbocation.
Base removes H+ from adjacent carbon.
Pi bond forms. 59
Draw both of substitution and elimination mechanism of following reaction:
CH3
H3C Br
CH3
and CH3OH
60
SUMMARY OF E1 REACTIONSSUMMARY OF E1 REACTIONS
Substrate : benzyl > allyl > ~3 RX > 2 RX (1 RX and CH3X are unlikely)
Nucleophile: weak base
Solvent: polar protic solvent
Kinetics: first-order rate equation, kr [RX]
Stereochemistry: no particular geometry
Rearrangement: common to form most stable carbocation
Product follows Saytzeff’s rule (alkene) 61
Saytzeff’s RuleSaytzeff’s Rule
If more than one elimination product is possible,
the most-substituted alkene is the major product
(most stable).
R2C=CR2>R2C=CHR>RHC=CHR>H2C=CHR
tetra > tri > di > mono
C C
Br
H
C
H
CH3
H
H
H
CH3
OH-
C CH
HC
H H
CH3
CH3
C
H
H
H
C
H
CCH3
CH3
+
minor major
62
Bimolecular elimination
Requires a strong base
Halide leaving and proton abstraction happens
simultaneously - no intermediate.
Second-Order Elimination: Second-Order Elimination: The E2 ReactionThe E2 Reaction
63
E2 MechanismE2 Mechanism
64
65
Stereochemistry of E2Stereochemistry of E2
66
SUMMARY OF E2 REACTIONSSUMMARY OF E2 REACTIONS
Substrate : benzyl > allyl > ~3 RX > 2 RX (poor SN2 substrate)
(1 RX and CH3X are unlikely)
Nucleophile: strong base
Solvent: polarity is not so important
Rearrangement: none
Product follows Saytzeff’s rule (alkene)
67
Substitution or EliminationSubstitution or Elimination
Strength of the nucleophile determines order:
Strong nuc. will go SN2 or E2.
Primary halide usually SN2.
Tertiary halide mixture of SN1, E1 or E2
High temperature favors elimination
Bulky bases favor elimination
Good nucleophiles, but weak bases, favor
substitution.
68
Strong Bases Weak Bases
Good Nucleophiles HO-, RO- (if R is not bulky)RCC:-
I-, Br-, RS-, CN-, N3-
Poor Nucleophiles K+ -OC(CH3)3
other hindered alkoxidesCl-, F-, RCO2
-
H2O, ROH, RCO2H
Type of Alkyl Halide Nucleophile/Base Mechanism 1o strong or weak, non-bulky SN2 1o strong and bulky E2(Note: even with a non-bulky base, it is also possible to force elimination in 1o by heating)
2o good nucleophile, weaker base SN2 (e.g. RS-, I-) 2o weak nucleophile, weak base SN1 (e.g. H2O, ROH) 2o strong base, weaker nucleophile E2 (e.g RO-) 69
3o weak nucleophile, weak base SN1 and some E13o stronger base or stronger nucleophile E2
(Notes : SN2 never occurs for 3o because of steric hindrance; carbocations are formed easily in 3o halides)
Type of Alkyl Halide Nucleophile/Base Mechanism
70
Visual tests for alkyl halides
a) Reagent AgNO3 / ethanol
This reaction proceeds by abstracting a halogen by the Ag+ to form carbocation. Therefore it is an SN1 type of reaction. Thus we can use this reaction to differentiate classes of alkyl halides & types of halogen.
Since SN1: benzylic~ allylic> 3o > 2o > 1o.
CH2X+ AgNO3
ethanolAgX + CH2+
+ AgNO3AgX +CH2=CH CH2X CH2=CH CH2
+
71
Observations:
i) Immediate precipitation with benzylic, allylic
and 3o R-X.
ii) Precipitate formed after 5 mins with 2o R-X.
iii) No reaction with aryl or vinyl R-X!
iv) This reagent can also differentiate between the type of
halogens present in the halides.
Eg: AgCl = white, AgBr & I = pale yellow72
b) Reagent NaI/ acetone
This reaction proceeds by displacement of bromide ion by the I- ions (strong Nu-), thus it is an SN2 type of reaction. Therefore is used to differentiate classes of R-Br.
Sequence of reactivity:
1o > 2o> 3o. No reaction with aryl or vinyl R-X.
R-CH2-Br + NaI R-CH2-I + NaBr (precipitate)
R2-CH-Br + NaI precipitate after 5 minutes
R3-C-Br + NaI very slow 73
I) Is there strong base?
If YES, there is strong base:A) There will be no E1/SN1.B) The products will come from E2 and/or SN2, depending upon the halide:
1) 1º halides give SN2 unless they are very hindered on the back side or the base is very hindered (a poor nucleophile). With potassium tert-butoxide you get a good yield of E2 products.2) 3º halides give pure E2.3) 2º halides give a mixture of E2 and SN2 products—mostly E2.
II) If there is a weak base and a good nucleophile?
If YES, there is a weak base/good nucleophile:A) 1º and 2º alkyl halides will give SN2. 3º halides will give mostly SN1/E1.
III) If there is no good nucleophile and no strong base:
1) 1º halides give SN2. This SN2 is very slow with H2O, ROH, or RCO2H2) 2º and 3º halides give E1/SN1.
74
Give IUPAC name for the C4H9I isomers and classify them whether they are 1o,2o or 3o
CH3CH2CH2CH2I
1-iodobutane
CH3CH2CHICH3
2-iodobutane
CHCH2I
CH3
H3C
1-iodo-2-methylpropane
C
CH3
H3C
CH3
I
2-iodo-2-methylpropane
75
Predict the products and mechanisms of the following reactions.
1. Ethyl bromide + sodium ethoxide2. t-butylbromide + sodium ethoxide3. Isopropyl bromide + sodium ethoxide4. Isobutylbromide + potassium hydroxide in ethanol/ water
76