cyclohexane n.
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cyclohexane. cis -. 1,2-. dimethyl. cyclo. hexane. cis -1,2-dimethylcyclohexane. axial - equatorial. trans -. 1,2-dimethylcyclohexane. trans -1,2-dimethylcyclohexane. axial - axial. equatorial - equatorial. Optical isomerism. Stereocenter 4 different substituents. C * =. *. Br.

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cyclohexane
cyclohexane

cis-

1,2-

dimethyl

cyclo

hexane

cis-1,2-dimethylcyclohexane

axial-equatorial

trans-

1,2-dimethylcyclohexane

trans-1,2-dimethylcyclohexane

axial-axial

equatorial-equatorial

optical isomerism
Optical isomerism

Stereocenter

4 different

substituents

C*=

*

slide3

Br

Br

Fischer projections

*

C

C

*

I

Cl

Cl

I

H

H

(+)

(-)

Br

Br

Enantiomers

*

*

I

Cl

Cl

I

H

H

non-superimposable mirror images

enantiomers
Enantiomers

identical in most properties

differ in:

1.interaction with polarized light

2. interaction with chiral environments

Light interacts with molecules

when it passes through them

[interaction of electrical fields]

When light encounters mirror image

of molecule, interaction is reversed

polarized light
Polarized light

(-)

Oneenantiomer -

rotate light to the left

(+)

Otherenantiomer -

rotate light to the right

in 50/50 mix -

no net rotation

racemic mixture

one enantiomer -

polarized light will be rotated

optical activity

optical isomerism1
Optical isomerism

Stereocenter

4 different

substituents

C*=

*

*

3-methyl

hexane

bromo

chloro

iodo

methane

*

*

bromo

cyclo

pentane

trans-

1,3-dibromo

cyclopentane

no C*

alkane summary
Alkane Summary

1. Alkanes -sp3 hybridized

2. Relatively unreactive

Substitution with halogens

Combustion

3. Non-polar

IMF = London Dispersion Forces

size

structure

slide8

4. Free rotation around C-C bonds

conformations

5. Non-cyclic alkanes - structural isomers

6. Cyclic alkanes - geometric isomers

cis-, trans-

7. Alkanes -optical isomers

stereocenters

C*

slide9

alkyl halides

1o

uv

+

Cl2

1o

3o

1o

2o

..

..

:Cl:Cl:

..

..

..

2 :Cl.

half-arrow = 1e-

step 1

..

form mostly

stability of free radicals

3o > 2o > 1o

slide10

3-methylhexane

no reaction

+ OH-

*

stereocenter

*

+ Br-

+ OH-

C+

electrophile

e- deficient

+

..

-

:

OH-

nucleophile

e- rich

..

slide11

Nucleophilic Substitution

SN1

*

*

+ Br-

+ OH-

reaction is 1st order in C7H15Br

zero order in OH-

reactants are optically active

rate = k [R – X]

products are optically inactive

slide12

CH3

CH3

. .

. .

:

:

C2H5 – C – Br

:

C2H5 – C+ Br-

. .

. .

C3H7

C3H7

Nucleophilic Substitution

SN1

rate determining step unimolecular

step 1

slow

C+

carbocation

(4 – ½ (6) – 0) = +1

reactants are optically active

rate = k [R – X]

products are optically inactive

slide13

CH3

CH3

. .

. .

:

:

C2H5 – C – Br

:

C2H5 – C+ Br-

. .

. .

C3H7

C3H7

. .

CH3

. .

:

- O – H

. .

:

H – O -

. .

C+

CH3

CH3

C3H7

HO

C3H7

H7C3

OH

C2H5

C2H5

C2H5

Nucleophilic Substitution

SN1

step 1

slow

step 2

*

*

reactants are optically active

rate = k [R – X]

products are optically inactive

slide14

E

C+

Nucleophilic Substitution

SN1

Ea

carbocation intermediate

1st order in R – X

2 products

Nu – R + X-

Nu-

+ R – X

slide15

Nucleophilic Substitution

SN2

*

*

+ Br-

+ OH-

reaction is 1st order in C4H9Br

1st order in OH-

rate = k [R – X]

[OH-]

reactants are optically active

products are optically active

slide16

H

H

H

. .

. .

:

C – Br

HO – C – CH3

H-O

. .

:

---C---Br

. .

CH3

C2H5

C2H5

CH3

C2H5

. .

:

H – O -

. .

C2H5

C2H5

. .

:Br:-

. .

H

Br

HO

H

CH3

CH3

Nucleophilic Substitution

SN2

rate determining step bimolecular

step 1

*

slow

transition state

stereocenter inverted

reactants are optically active

rate = k [R – X][OH-]

products are optically active

slide17

 -

 -

Nu---C---X

E

Nucleophilic Substitution

SN2

Ea

1st order in R – X

1st order in Nu-

1 product

Nu – R + X-

Nu-

+ R – X

slide18

SN1 or SN2

H

R

R

R

H

R

3o R –X

SN1

carbocation

3o > 2o > 1o

yes

2o R –X

?

1o R –X

no

slide19

SN1 or SN2

1o R – X

yes

SN2

transition state

2o R –X

?

3o R – X

no

slide20

SN1 or SN2

3o R –X

SN2

1o R – X

yes

yes

SN1

2o R –X

2o R –X

?

?

1o R –X

no

3o R – X

no

Other factors influencing rates:

Nucleophile :

SH-

I-

CN-

charged

OH-

neutral

NH3

H2O

Solvent

Temperature