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Alkenes: Structure, Nomenclature, and Reactivity, Schemes and Mind Maps of Stereochemistry

An in-depth exploration of alkenes, their structure, nomenclature, and reactivity. It covers topics such as the naming of alkenes, their physical properties, the formation of cis and trans isomers, and their reactions with various electrophiles and nucleophiles. The document also discusses the role of electrophilic and nucleophilic reagents in the addition reactions of alkenes.

Typology: Schemes and Mind Maps

2021/2022

Uploaded on 09/27/2022

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Alkenes: Structure and Reactivity
Alkenes, also called ______, are hydrocarbons containing
one or more double bonds.
olefins
Naming the alkenes:
Rule 1
: Find the longest chain containing the double bond and name the
Rule
1
:
Find
the
longest
chain
containing
the
double
bond
and
name
the
compound by replacing the alkane suffix –ane with –ene.
Rule 2: Number carbons in the chain so that double bond carbons have the
lowest possible numbers; if the double bond is equidistant from two
ends, begin at the end nearer the first substituent.
8-1
Nomenclature of Alkenes
vinylic carbons
Hex-2-ene 2-Methylhex-3-ene
allylic carbons
vinyl
2
Eth l
t
1
2
-
Methyl
buta
-
-
diene
_____ group
gro p
vinyl
allyl
2
-
Eth
y
l
pen
t
-
1
-ene
2
-
Methyl
buta
-
,
-
diene
-diene and -triene
Cycloalkenes:
_____
gro
u
p
allyl
8-2
Factoids of Alkenes
Polarization of Alkenes
m.p. -139 oC
b.p. 4 oC
m.p. -106 oC
b.p. 1 oC
Th b i i i
l
Thi l d h ibili f
i
d
8-3
Th
e
b
arr
i
er to rotat
i
on
i
s very
l
arge.
Thi
s
l
ea
d
s to t
h
e poss
ibili
ty o
f
c
i
san
d
trans ________, stable, separable compounds with different properties.
isomers
E,ZNomenclature
Sometimes “cis” and “trans” are ambiguous;
in such cases we resort to the more formal
cis or trans?
Z
Steps to assign
E
and
Z
descriptors
in
such
cases
we
resort
to
the
more
formal
E,Znomenclature.
Z
Steps
to
assign
E
and
Z
descriptors
For each C atom, assign substituent priority
Zif highest priority substituents on same side of double bond
Eif highest priority substituents on opposite side of double bond
hi h
l
hi
g
h
er
l
ower
lower hi
g
her
higher higher
lower
lower
g
lower
lower
8-4
pf3
pf4
pf5
pf8
pf9

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Alkenes: Structure and Reactivity^ Alkenes

, also called ______, are hydrocarbons containing

one or more double bonds.

olefins

Naming the alkenes:^ Rule 1

: Find the longest chain containing the double bond and name the Rule

1: Find the longest chain containing the double bond and name thecompound by replacing the alkane suffix –ane with –ene. Rule 2: Number carbons in the chain so that double bond carbons have the

lowest possible numbers; if the double bond is equidistant from twoends, begin at the end nearer the first substituent.

8-

Nomenclature of Alkenes

vinylic carbons

Hex-2-ene

2-Methylhex-3-ene

allylic carbons

vinyl

2 Eth l

t

1

2 - Methyl

buta

- 1 3-

diene

_____ group

gro p vinyl allyl

2 -Ethylpent-1-ene

2-Methylbuta-1,3-diene- diene

and -

triene

Cycloalkenes:

allyl _____ group

Factoids of Alkenes

Polarization of Alkenes

m.p. -

o C b.p. 4

o C

m.p. -

o C b.p. 1

o C

Th^

b^

i^

i^

i^

l^

Thi^

l^ d

h^

ibili^

f^ i^

d^ 8-

The barrier to rotation is very large. This leads to the possibility of

cis^

and

trans

isomers ________, stable, separable compounds with different properties.

E ,

Z^

Nomenclature

Sometimes “

cis ” and “

trans

” are ambiguous;

in such cases we resort to the more formal

cis^

or^ trans

Z

Steps to assign

E^ and

Z^ descriptors

in such cases we resort to the more formal E , Z

nomenclature.

Z

Steps to assign

E^ and

Z^ descriptors

  • For each C atom, assign substituent priority•^ Z^

if highest priority substituents on same side of double bond

-^ E^

if highest priority substituents on opposite side of double bond

hi^ h

l

higher

lower

lower

higher

higher

higher

lower

lower

g

lower

lower

Stability of Alkenes

8-

more hyperconjugation

Heat of Hydrogenation: a Measure of Stability

o ∆ H

Preparation of Alkenes

follows Zaitsev’s rule

Elimination Reactions:

An E2 reaction that generatesthe thermodynamically lessfavored isomer is said tofollow the

Hofmann rule follow the

Hofmann

rule.

+ H

Dehydration of Alcohols:

+ H

8-

Reactivity in dehydration:o^1

o< 2 < 3

o^ alcohols

Reactions of Alkenes: The C=C Double

Bond as a NucleophileBond as a Nucleophile

A pi bond can donate electronsp bo d ca

do a e e ec

o s

This atom has ____________from

(^2) sp to^ sp

3

re-hybridized

The brackets denote

This carbon now has an empty

The

brackets denote an ___________ alongthe reaction pathway

intermediate

p orbital; its incomplete octet(6 electrons) makes it electrondeficient

Product Distribution and

the Reaction Coordinate Diagramthe Reaction Coordinate Diagram

For this step the TS

≠^ is

For^

this step, the TS

is

late. Thus, we orderedthe energies of the TS

≠s

according to the energiesaccording to the energiesof the intermediates.

8-

Electrophilic Addition: Alkyl Halide Preparation^ •

The reaction is successful with HCl and with HI as well as HBr• HI is generated from KI and phosphoric acid• HI is generated from KI and phosphoric acid Predict the relative reactivity of the following alkenes towardelectrophilic addition of HX and give your explanations.

(CH

)C=CH 3 2

2

CH

CH=CHCH 3

3

CH

CH=CH 3

2

CH

=CH 2

2

(CH

)C=CH 3 2

CH 2

CH=CHCH 3

CH 3

CH=CH 3

CH 2

=CH 2

2

Rearrangement of Carbocations

8-

Electrophilic Hydration of Alkenes

Alkene hydration and alcohol dehydration are equilibrium processes.

mechanism^ Acid Catalyzed Equilibration of Alkenes

y^

q

catalytic^ H

+

catalytic H

+^

more stable; major componentin isomeric product mixture

Electrophilic Addition of Halogen to Alkenes

8-

a racemic mixture (

± )

Stereochemistry Elucidates the Mechanism

- Br

a carbocation wouldbe trigonal planar

O t t

ti fi d f

ll^ t

Octet satisfied for all atoms

Bromonium Trapped by Other Nucleophiles

( ± )

Br^2 CH^ OH^3

( ± ) ( ± )

similarly followingMarkovnikov rule

8-

Regioselectivity in Halonium Ion Opening

Carbene and Carbenenoid Reactions

bicyclo[4.1.0]heptane

cis -diethylcyclopropane

Simmons-Smith reagent

: (iodomethyl)zinc iodide

“^ b

id

” (^

b^

lik^

b t^

)

carbenoid

”^ (a carbene-like substance)

8-

92%

Reduction of Alkenes: Catalytic Hydrogenation

syn-

addition

1

diastereoselective diastereoselective

Oxidation and Reduction in Organic

ChemistryChemistry

Reduction

increase electron density on carbon by:

Reduction

increase electron density on carbon by:— forming: C–H— or breaking: C–O

C–N

C–X

Oxidation

decrease electron density on carbon by:— forming: C–O

C–N

C–X

— or breaking: C–H— or breaking: C–H

8-

an epoxide

Epoxide Synthesis electrophilic

  • δ

oxidation by

peroxycarboxylic acid:

via halohydrin:

Diol Formation: Epoxide Ring-Opening

vicinal

anti dihydroxylation

8-

Vicinal

Syn

Dihydroxylation of Alkenes

Not very useful forsynthesis: easily over-oxidized into ketones

cold, dilute(w/ NaOH)

oxidized into ketonesand/or carboxylic acids.

Use of OsO

or NaHSO

3

Use

of OsO

+ OsO+^ OsO

3

  • OsO

3

8-

a^ syn

vicinal diol

OsO

: expensive and toxic 4

Oxidative Cleavage: Ozonolysis

( or^ CH

SCH 3

) 3

mechanism:

8-

More Oxidative Cleavages to Carbonyl

CompoundsCompounds

KMnO

4

+ H

4

HIO

4

or NaHSO

3

8-