Reaction Types

additions 
A + B --> C 
H-Cl + CH2=CH2 --> CH3-CH2-Cl

eliminations 
X --> Y + Z 
CH3-CH2-Cl --> CH2=CH2 + H-Cl

substitutions 
A-B + C-D --> A-C + B-D 
CH4 + Cl2 --> CH3-Cl + H-Cl

rearrangements 
X --> Y 
cyclopropane --> propene

Reaction Mechanisms

a step-by-step account of how a reaction occurs (and why)

bond-breaking steps: 
homolytic: one electron to each fragment 
(generates radicals)
heterolytic: both electrons to one fragment 
(generates ions)

bond-making steps: 
homogenic or heterogenic

Electron Movement

electron-pushing arrows indicate the flow of electrons in a mechanism

electrophile: 
electron-deficient species seeking a pair of electrons (a Lewis acid )

nucleophile: 
electron-rich species that can provide a pair of electrons (a Lewis base)

HCl plus Ethene

CH2=CH2 + H-Cl --> CH3-CH2-Cl

an electrophilic addition

reaction type: addition

reagent type: an electrophile 
HCl, actually H+, a strong Lewis acid

Addition Mechanism

pi bond is relatively reactive, especially towards electrophiles
it provides a good source of electrons

addition of H+ to CH2=CH2 forms a new C-H sigma bond
the electrons for the new bond came from the pi bond
the other C is left with only 6 e-

Carbocation Intermediate

an intermediate is formed in the reaction mechanism
CH2=CH2 + H+ --> CH3-CH2+

carbocation: a carbon atom with only 3 bonds (6 e-) and a positive charge
structure: sp2 hybridized (trigonal)

Formation of Chloroethane

the reaction is completed as chloride anion (a nucleophile) adds to the carbocation (an electrophile)
CH3-CH2+ + Cl- ---> CH3-CH2-Cl

Equilibrium

for a general reaction: 
A + B <==> C + D

equilibrium constant, K 
K = [C] [D] / [A] [B]

favorable reactions have large K

unfavorable reactions have small K

Kinetics

rates of reaction

may or may not correlate with the favorability of the equilibrium

rate depends on the mechanism

is there a good way to get from reactants to products?

Heats of Reaction

delta H (enthalpy change) 
delta H = H(products) - H(reactants)

exothermic means delta H <0 (negative) 
reaction gives off heat

endothermic means delta H >0 (positive) 
reaction absorbs heat

Potential Energy Diagrams

draw exothermic reactions downhill

draw endothermic reactions uphill

Activation Energy, Ea

there is usually an energy barrier between reactants and products

activation energy represents the highest amount of energy necessary while travelling along the minimum-energy (easiest) pathway from reactants to products

The Transition State

structure of the molecule(s) at the highest point along the reaction pathway

the stability of the transition state (relative to reactants) determines Ea (rate of reaction)

Alkene Addition Reactions

pi bonds undergo addition reactions
CH2=CH2 + HCl --> CH3CH2Cl

in general,
C=C + HX --> H-C-C-X

alkenes react with hydrogen halides to form alkyl halides

Addition of HX to Alkenes

cyclohexene + HBr --> bromocyclohexane

1-methylcyclohexene + HBr --> 1-bromo-1-methylcyclohexane 
(not 1-bromo-2-methylcyclohexane)

Reaction Notation

reactants -------> products
focus on the organic reactants and products

show reagents over the arrow

show solvent and conditions under the arrow
(or show full balanced reaction)

Orientation of Addition

regiochemistry:|
specific orientation of addition
(which C gets H, which gets X?)

alkene additions are regioselective:
one direction of addition is usually preferred

Markovnikov's Rule

the original:
add H to the C with more H's
(or to the C with fewer alkyl groups)

the reason:
add H+ to form the more stable cation
CH3CH=CH2 + HCl ---> 
CH3CH+CH3 (not CH3CH2CH2+)
---> CH3CHClCH3 (not CH3CH2CH2Cl)

Carbocations 

structure: trigonal (sp2)

stability: 3° >2° >1°

more alkyl groups stabilize a cation by electron donation to the electron-deficient (6-electron) carbocation

Markovnikov Addition


Hydration of Alkenes

alkene + water --> alcohol
CH2=CH2 + H2O --(H+)--> CH3CH2OH

mechanism:

step 1: 
addition of H+ electrophile to pi bond

step 2: 
addition of H2O nucleophile to cation

Hydration Mechanism


Halogenation of Alkenes

CH2=CH2 + Cl2 ---> Cl-CH2-CH2-Cl

mechanism:
Cl2 is an electrophile (adds Cl+)
then Cl- is a nucleophile

Anti Addition

anti stereochemistry: 
two new groups are added to opposite sides of the original pi bond

cyclopentene + Br2 ---> trans-1,2-dibromocyclopentane (no cis)

anti - describes the process

trans - describes the product

Bromonium Ion

carbocations can be stabilized by bonding to a neighboring Br
(also works with Cl, but less favorable)

Reduction of Alkenes

reduction - addition of H2
(or removal of O)

CH2=CH2 + H2 ---> CH3-CH3

R-O-H + H2 ---> R-H + H2O

Catalytic Hydrogenation

CH2=CH2 + H2 ---> CH3-CH3

requires an active catalyst, typically Pt, Pd, Ni, PtO2

reaction occurs on the surface

both Hs are delivered to the same side of the pi bond

Syn Addition

syn stereochemistry: two new groups are added to the same side of the original pi bond

1,2-dimethylcyclohexene + H2 --(cat)-->cis-1,2-dimethylcyclohexane(no trans)

syn - describes the process

cis - describes the product

Oxidation of Alkenes

oxidation - addition of O
(or removal of H2)

RCH2OH ---> RCH=O ---> RCOOH

there are a wide variety of oxidizing agents:
O2, O3, KMnO4, CrO3, Na2Cr2O7
metals in high positive oxidation states

Hydroxylation

alkene + KMnO4 --(base)--> 1,2-diol

addition of two OH groups is syn

cyclopentene --> cis-1,2-cyclopentanediol

Oxidative Cleavage

C=C --> C=O + O=C 

acidic KMnO4 causes cleavage

ozone (O3) causes cleavage

sometimes useful degradation method to identify unknown compounds

Balancing Redox Reactions

identify the two half-reactions
- what gets oxidized, what gets reduced?

balance all elements except O and H

balance O 
use H2O (in acid) or OH- (in base)

balance H 
use H+ (in acid) or H2O (in base)

balance charge with electrons (e-)

Polymers

long chains of repeating units (monomers)

n CH2=CH2 --(init)--> (init)-(CH2-CH2)n-

n=100-10,000 polyethylene

has properties like a very long alkane

many polyalkenes are commercially important materials and plastics
e.g., PVC, Teflon, Orlon

Chain Reactions

polymerization occurs by a free radical chain mechanism

initiation - generation of the first free radical from an initiator
R-O-O-R --(heat)--> 2 R-O·
(initiators have one weak bond)

Chain Reactions

propagation - radical adds to a p bond
RO· + CH2=CH2 ---> RO-CH2-CH2·

note that the product is also a radical
RO-CH2-CH2· + CH2=CH2 ---> RO-CH2-CH2-CH2-CH2· ---> etc.

typically this occurs hundreds or thousands of times
(until radicals recombine - termination)

Substituted Monomers

radical additions follow the Markovnikov Rule:
add radicals to form the more stable radical intermediate

radical stability is like cation stability: 3° >2° >1°
this leads to polymers with alternating substituents

Vinyl Polymers

polyvinyl chloride

polypropylene

polystyrene

Alkyne Additions

similar to alkenes but more reactive

Markovnikov Rule is followed

excess reagent gives double addition

single addition is usually possible

single addition gives alkene product, which may be cis (syn addition) or trans (anti addition) or nonspecific

Reduction of Alkynes

excess H2 + catalyst gives alkanes