ORGANIC CHEMISTRY 354

ALKENE STABILITY

ELECTROPHILIC ADDITION OF HX TO ALKENES

MARKOVNIKOV'S RULE

CARBOCATION STABILITY

ALKENE STABILITY!

WHICH IS MORE STABLE?

THE TRANS STEREOISOMER IS MORE STABLE THAN THE CIS STEREOISOMER SINCE THE TRANS DOES NOT HAVE THE STERIC INTERFERENCE (STRAIN) THAT THE CIS DOES!

cis-2-BUTENE --?--> trans-2-BUTENE
---and---
trans-2-BUTENE --?--> cis-2-BUTENE

THESE REACTIONS DO NOT OCCUR READILY!

BUT, WITH AN ACID CATALYST

CAN'T ALWAYS EQUILIBRATE

NEED ANOTHER WAY TO DETERMINE RELATIVE ENERGIES

CONVERT THE TWO ALKENES IN QUESTION TO A COMMON PRODUCT AND COMPARE ENERGIES

a. HYDROGENATION

TRANS-2-BUTENE + H2 ---> BUTANE + 27.6 kcal/mol

CIS-2-BUTENE + H2 ---> BUTANE + 28.6 kcal/mol

1-BUTENE + H2 ---> BUTANE + 30.1 kcal/mol

BOTH CIS- AND TRANS-2-BUTENE ARE MORE STABLE THAN 1-BUTENE

CAN WE COMPARE 1-BUTENE AND ISOBUTYLENE?

b. COMBUSTION!

1-BUTENE + 6 O2 ---> 4 CO2 + 4 H2O + ENERGY

ISOBUTYLENE + 6 O2 ---> 4 CO2 + 4 H2O + ENERGY

ENERGY DIFFERENCE IS ABOUT 1.7 kcal/mol WITH ISOBUTYLENE BEING LOWER IN ENERGY

GENERALIZATION

WHICH IS MORE STABLE?

Answer

ALKYL GROUPS STABILIZE AN ALKENE MORE THAN A HYDROGEN DOES.

THIS STABILIZATION MORE THAN OFFSETS THE STERIC EFFECT OF TWO ALKYL GROUPS CIS TO EACH OTHER.

WHICH IS MORE STABLE?

Answer

WHICH IS MORE STABLE, (E)-4-METHYL-2-HEPTENE or (E)-4-METHYL-3-HEPTENE?

ELECTROPHILIC ADDITION TO ALKENE

REACTION IS OFTEN RUN IN ETHER SOLVENT

WRITE IN DIFFERENT WAYS DEPENDING ON EMPHASIS.

IS THE REACTION GENERAL, WILL IT WORK WITH OTHER ALKENES?

IN ALL CASES THE HALOGEN WENT TO THE MORE HIGHLY SUBSTITUDED CARBON.

REACTION IS REGIOSELECTIVE

WHEN A REACTION WHICH CAN POTENTIALLY YIELD TWO OR MORE CONSTITUTIONAL ISOMERS ACTUALLY PRODUCES A PREPONDERANCE OF ONE, THE REACTION IS SAID TO BE REGIOSELECTIVE.

NOTE

MARKOVNIKOV'S RULE!

IN THE IONIC ADDITION OF ACID HX TO AN ALKENE, THE HYDROGEN OF THE ACID ATTACHES TO THE CARBON OF THE ALKENE WHICH HAS THE LARGER NUMBER OF HYDROGENS.

PREDICT THE PRODUCT:

Answer

Answer

WHICH ALKENE IS NEEDED TO SYNTHESIZE:

Answer

WHY DO WE HAVE MARKOVNIKOV'S RULE?

IT APPEARS THAT TERTIARY CARBOCATIONS ARE MORE STABLE THAN SECONDARY CARBOCATIONS WHICH ARE MORE STABLE THAN PRIMARY CARBOCATIONS.

3o > 2o > 1o > METHYL CARBOCATION

OTHER EVIDENCE:

STABLE SALTS OF (CH3)3C+1 HAVE BEEN MADE BUT NOT CH3CH2CH2CH2+1

TAKES LESS ENERGY TO MAKE (CH3)3C+1 FROM (CH3)3CCl THAN CH3+1 FROM CH3Cl

STRUCTURE OF METHYL CARBOCATION

WHY ARE TERTIARY CARBOCATIONS MORE STABLE THAN SECONDARY, ETC?

INDUCTIVE EFFECT

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