Chapter 6 Reactions of Haloalkanes: SN2
I.
Naming Haloalkanes
A.
B.
Treat Halogen as a substituent to an alkane: chlorine = chloroName as in alkane nomenclature
F
I
Br
Cl
2-bromo-2-methylpropane
II.
cis-1-chloro-2-fluoro
cyclohexane
4-(1-iodoethyl)heptane
Physical properties of haloalkanes
A.
Bond Strength
1) For RCH2X, F > Cl > Br > I bond strength
2) F has best size for overlap
3) Bond Length: I > Br > Cl > F
B.
Polarity: Halogens are more electronegative than C, so bonds are polar
1) Polar R-X bond can be attacked by cations or anions
2)
+
-
d
-
d
d
R
X
X
R
II.
+
d
Polarizability = how much the electron cloud is deformed by other
charges, combination of size and charge effects
a) I > Br > Cl > F because of larger size and same charge (-1)
b) London Forces are largest for Iodoalkanes because of polarizability
c) Boiling points: MeI > MeBr > MeCl > MeF > Methane
Nucleophilic Substitution
A.
Nucleophile/Electrophile
1) Molecule or Ion with a lone pair of electrons seeks to share or donate their
“extra” electron density to a (+) or (d+) “nucleus”
a) Nucleophile = nucleus (+) loving = Nu
b) Nucleophiles = Lewis Bases
2)
Molecule or Ion having (+) or (d+) region seeks extra electron density
a) Electrophile = electron (-) loving
b) Can be a Lewis Acid, but not necessarily
c) Just needs d+, so any polar bond should have one electrophilic area
d) An atom with a full octet can still be an electrophile
d+
R
dX
B.
Nucleophilic Substitution Reaction
1)
-
2)
3)
4)
+ X-
Nu
+ R
X
Nu
R
Nu
+ R
X
Nu
R+ + X-
Leaving Group = ion or molecule replaced by Nu (X) “Substitution”
Color codes in your book: Nu = red, Electrophile = blue, LG = green
Examples
-
HO + CH3
Cl
OH + Cl
CH3
CN- +
+ II
NH3
C.
+
CN
+
Br
Electron Pushing in Mechanisms
1) Move e- from e- rich to e- poor sites
2) Acid-Base example H2O + HCl
NH3
-
Br
H3O+
+
Cl-
a)
b)
c)
3)
O lone pair become O—H bond
H—Cl bond becomes Cl lone pair
Curved arrows show “flow” of the electrons
Organic mechanisms, same thing: Must Draw Correct Lewis Structure
a) Nucleophilic Substitution
-
HO
b)
+ CH3
CH3+
I
+
I-
Nucleophilic Addition
O-
O
OH-
d)
OH
Dissociation
CH3
c)
CH3
Cl
+
R
HO
R
R
R
Electrophilic Addition
C C
+
+
H
H
+
C C
+ Cl-
D.
Kinetics tells us about mechanisms of Nucleophilic substitution
1) Nucleophilic substitution is 2nd order process: rate = k[OH-][CH3Cl]
-
OH
2)
3)
+ CH3
Cl
CH3
OH
+ Cl-
a) If double [OH-], double rate
b) If double [CH3Cl], double rate
c) Rate depends on both S.M., they must both be in rate determ. step
Bimolecular Process
SN2 = Bimolecular Nucleophilic Substitution
a) Concerted: bond-making and bond-breaking happen simultaneously
b) Front-side or Back-side displacement?
Front-side attack
Back-side attack
c)
For Achiral reactants, products are identical (No Help!)
d)
e)
f)
What about for Chiral reactants?
SN2 gives Stereospecific Inversion of Stereochemistry
SN2 happens exclusively through Back-side attack