unit vi (6) Addition Reactions of Alkenes

Alkenes have p-orbitals defining the

second bond; with electron density above and below the trigonal planar molecule

addition reactions occur from

top or bottom faces

addition reactions involve adding

AB species across the double bond without losing anything

addition reactions to alkenes involve a net addition that changes

hybridization from sp2 to sp3

hydrogenation is the

addition of H2

hydrogentation allows the

preparation of saturated hydrocarbons from alkenes

Metal catalysts facilitate the

reaction in heterogeneous conditions with a solid catalyst

metal catalyst mechanism involves H2 binding to

metal surface and syn addition, delivering two hydrogens to one face simultanesly

hydrogenation sterochemistry is 

100% syn addition in cyclohexane systems 

more stable alkenes have lower heats of

hydrogenation (becuase they start with less PE)

What is the trend of alkene stability (most to least)

monosubstituted > disubsit cis > disub trans > trisubsti > tetrasubst

Pi electrons create regions

of high electron density above and below the trigonal planar carbons; making alkenes nucleophilic 

electrophiles have region of positive charge that 

react with nucleophilic double bonds 

markovnikov’s rule

hydrogen adds to the carbn with more hydrogens

Markovnikov’s rule reflects

carbocation stability: first step generates most susbtituated carbon

the alkene attacks h-X forming a

carbocation that is then trapped by the halide anion. primary carbocations don’t form, ensuring regioselectivity

carbocation rearrange rapidly via 

hydride or methyl shifts to form stable species 

achiral reagents give

achiral products

When chiral centers form

50-50 racemic mixtures result

strong acids form h30+ in

water

mechanism involves carbocation formation followed by

water attack and deprotonation 

each step is

fully reversible

hydration and dehydration are

two sides of an equilibrium

adding water drives

hydration

removing water drives 

dehydration 

removing volatile alkenes drives

elimination

in addition reactions the

alkene acts as the nucleophile

the carbocation is a

strong electrophile due to its positive charge

the equilibrium between addition and elimination depends on 

reaction conditions and relative bond strengths 

carbocation forms on the carbon that creates the most

stable carbocation

boron hydrides provide

nucleophilic hydrogen

hydroboration mechanism forms a

lewis acid-base complex where alkene attacks boron creating an intermediate with positive charges on the carbons

In hydroboration-oxidation 

hydrogen acts as a nucelophile

hydroboration-oxidation us a

syn-addition with oxidation occurring with retention of configuration

Halogenation

halonium ion formation

halogenation produces

anti-addition products through a halonium ion intermediate

in halogenation, the halonium ion undergoes sn2-like ring opening with

attack from the back face, producing anti-stereochemistry 

halohydrin formation

halogenation occuring in water: halonium ion is trapped by water forming a halohydrin

Epoxidation

peroxy acids covert alkenes to epoxides

peroxy acids

containing extra oxygen

epoxidation is a 

concerted syn-addition without carbocation intermediates 

ozonolysis

cleaves alkenes in carbonyl compounds

product of ozonolysis

split the double bond and attach oxygen to each carbon

SN2 conditions

Strong nucleophile, polar aprotic solvent, low temperature

E2 conditions 

Strong bulky base (e.g., potassium t-butoxide)

Hydroboration conditions

BH₃·THF followed by H₂O₂/OH⁻ → anti-Markovnikov, syn-addition

Halogenation

X₂ → anti-addition via halonium ion

Epoxidation

Epoxidation: Peroxy acid → syn-addition

Ozonolysis 

O₃ then Me₂S → cleaves double bond to carbonyls