EP101 Sen Lnt 003 Alkene May11

5/25/2018 EP101 Sen Lnt 003 Alkene May11

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AlkenesJully Tan

School of Engineering


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WK 3 EP101 / EG101 2School of Engineering

Alkenes: Names and Structures Alkenes- hydrocarbons that have one or more double bonds between carbon atoms

CnH2n

Monounsaturated- one double bond

DieneAlkenes that have two double bonds.

Polyunsaturated- two or more double bonds

Double bonds (alkenes) is shorter than single bond (alkanes).

Each pi bond (and each ring) decreases the number of Hs by two.

Each of these is an element of unsaturation.


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Nomenclatureof Alkenes


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Nomenclature: One double bonding Alkenes are named using the IUPAC system

Rules:Step 1: Find the longest chain that includes bothcarbons of the double or triple

bond. Indicate the length of the parent chain by using a prefix that tell the

number of carbon atoms and the suffix, -ene(dropane)

Step 2: Number the chain from the end that gives the lower set of numbers to the

carbons of the double bond. Designate the position of the double bond by

the number of its 1st carbon.

Step 3: Branched Alkenes are named in a manner similar to alkanes; substituent

groups are located and named.

In naming cycloalkenes, we number the carbon atoms of the ring double bond 1and 2 in the direction that gives the substituent encountered first the lower

number.

CH3 - CH = CH - CH2 - CH(CH3) - CH3 is called 5-methylhex-2-ene


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Common Names Usually used for small molecules. Examples:

CH2 CH2

ethylene

CH2 CH CH3

propylene

CH2 C CH3

CH3

isobutylene

IUPAC: ethene propene 2-methylpropene


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The root name is based on the longest chain containingboth endsof allthe alkenes units.

The chain is numbered so as to give the one of the alkenes units the lowest possible number (i.e.

first point of difference).

The location for the lowest numbered carbon of each alkenes is used in the name.

The appropriate multiplier (i.e.di- for two, tri for three) is inserted before the -ene suffix.

In order make the name pronounceable, -a- is inserted after the root.

Nomenclature: Dienes, Trienes, and Polyenes


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1,4-pentadiene

2-methyl-1,3-butadiene

1,3-cyclopentadiene



Alkenes as Substituents In some cases, a group containing an alkenes may need to be treated as a substituent.

In these cases the substituent is named in a similar fashion to simple alkyl substituents.

The method is required when the alkenes is not the priority group. The substituent is named in a similar way to the parent alkenes.

It is named based on the number of carbon atoms in the branch plus the suffix-yl. i.e. alkenyl

There are two common names that are widely used:

Example CH3CH=CHCH(CH=CH2)2

1. Functional group is a alkenes, therefore suffix = -ene

2. The longest continuous chain with 2 C=C is C6 therefore root plus "a" =hexa

3. There are two alkenes in the parent chain, so insert the multiplierdi

4. Number from the right as drawn to give the C=C the lowest locants : therefore1- and 4-

5. The substituent is a C2 alkenyl group i.e. an ethenylgroup

6. The substituent is on C3

3-ethenylhexa-1,4-diene



Cis- and Trans- Terminology All terminal alkenes those with a C=CH2 unit do notexist as cis-and trans-isomers.

All 1,1-symmetrically disubstituted alkenes i.e. those with a C=CR2 unit do notexist as cis-and

trans-isomers.

Alkenes with the R-CH=CH-R unit canexist as cis-and trans-isomers.

If we consider the general alkenes unit shown below, then the alkenescan only exist as cisand trans

isomersif R1is not equal to R2ANDR3is not equal to R4.

If 2 identical groups attached to any one of the double bonded C, geometric isomerism is not possible.

33

3

CHCH

CH HCC

2-methyl-2-butene 1,1-dichloro-2-methyl-propene

Cl

ClCC

CH

CH

3

3



Name these:C C

CH3

H

H

CH3CH2

C C Br

H

Br

H

trans-2-pentene cis-1,2-dibromoethene



E- and Z-Nomenclature of Alkenes The cis- / trans- style is based on the longest chain whereas the E/Z style is based

on a set of priority rules.

For alkenes with 4 different substituents, we use E/Z nomenclature.

The E- and Z- style is more reliable and particularly suited to highly substituted

alkenes, especially when the substituents are not alkyl groups.

The Cahn-Ingold-Prelog priority rulesare used for naming geometric isomers(e.g. E- or Z-alkenes).

These rules are based on atomic number, and the first point of difference.
http://www.chem.ucalgary.ca/courses/350/orgnom/main/difference.htmlhttp://www.chem.ucalgary.ca/courses/350/orgnom/main/difference.html



Cahn-Ingold-Prelog Priority Rules Imagine each alkenes as two pieces, each piece containing one of the sp2 C

Assign the priority (high = 1, low = 2) to each group on each sp2 C based on atomic

number Determine the relative position of the two higher priority groups

If they are on the same side then it is a (Z)-alkenes (German; zusammen= together)

If they are on opposite sides then it is an (E)-alkenes (German; entgegen= opposite)

If there is more than one C=C that can be E/Z, then the location needs to be included

with the location, e.g.(2E,4Z)



Ranking Priorities: Cahn-Ingold-Prelog Rules Must rank atoms that are connected at comparison point Higher atomic number gets higher priority

Br > Cl > F > O > N > C > H

In this case,The higher prioritygroups are opposite:

(E )-1-bromo-1-chloro-propene



Example, -Z

C C

H3C

H

Cl

CH2C C

H

H

CH CH3

Cl1

2

1

2

2Z

2

1

1

2

5E

(2Z, 5E)-3,7-dichloro-2,5-octadiene



Boiling point - trends are similar to those shown in alkanes

- increases as they get more carbon atoms in their formula

more atoms = greater intermolecular Van der Waals forces

greater intermolecular force = more energy to separate molecules

greater energy required = higher boiling point

- for isomers, greater branching = lower boiling point

C2H4 (- 104 C) C3H6(- 48C) ....... C6H10(83C)

Melting point general increase with molecular mass

the trend is not as regular as that for boiling point.

Solubility alkenes are non-polarso are immiscible (dont mix with) with water

miscible with most organic solvents.

1. Physical Properties Of Alkenes



Geometry of C-C Double Bond AlkenesC bonded to 3 atoms, used sp2hybridized orbital.

In generally, double bond is shorter than single bond.

High electron density: pair of electrons inbond able to explain the reactivity of alkenes.

Rotation of C=C does not happen: any rotation of 1 C with respect of the other requires bond be

broken.

The rotation of C=C requires 2 C and 4 others atoms attached in the same plane, planar geometry.



Polarity alkenes is slightly polar than alkanes due to the electrons inbonds are more polarized. Alkyl

substituent donate a electron to C=C bond which increase polarization of vinylic bond.

Polarity is key in determining relative boiling pointas it causes increased intermolecular forces,

thereby raising the boiling point. In the same manner, symmetryis key in determining relative

melting pointas it allows for better packing in the solid state. Thus, trans-alkenes which are less

polar and more symmetrical have lower boiling points and higher melting points and cis-alkenes,

which are generally more polar and less symmetrical have higher boiling points and lower melting

points.

= 0.33 D = 0

cis-2-butene, bp 4C

C C

H

H3C

H

CH3

trans-2-butene, bp 1C

C C

H

H

H3C

CH3

Melting pt

Boiling pt Melting pt

Boiling pt



Comparing Stabilities of Alkenes Evaluate heat given off when C=C is converted to C-C.

This is partly due to their shape, the straighter shape of the trans isomer leads to hydrogenintermolecular forces that make the isomer more stable

Stability: cis < trans isomer

Less stable isomer is higher in energy, has a more exothermic heat of hydrogenation.

More stable alkenes gives off less heat

Trans butene generates 5 kJ less heat than cis-butene.



Substituent Effects More substituted alkeness are more stable.

H2C=CH2 < R-CH=CH2< R-CH=CH-R < R-CH=CR2< R2C=CR2

unsub. < monosub. < disub. < trisub. < tetra sub.

Alkyl group stabilizes the double bond.



Heats of hydrogenation of three butene isomers:

Overall Relative Stabilities of Alkenes

The greater the number of attached alkyl groups (i.e. the more highly substituted the

carbon atoms of the double bond), the greater the alkenes stability



2 Chemical Properties Of AlkenesELECTROPHILIC ADDITION MECHANISM

The main reaction of alkenes is addition

Because of the extra electron density in a C=C double

bond, alkenes are attacked by species which like

electrons.

These species are called electrophiles; they possess

a positive or partial positive charge somewhere in

their structure.



ELECTROPHILIC ADDITION MECHANISM

The electrophile, having some positive

character is attracted to the alkenes.

The electrons in the pi bond come out to

form a bond to the positive end.

Because hydrogen can only have two

electrons in its orbital, its other bondbreaks heterolytically. The H attaches to

one of the carbon atoms.

A carbocationis formed. The species thatleft now has a lone pair.

It acts as nucleophile and attacks the

carbocation using its lone pair to form a

covalent bond. Overall, there is ADDITION


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Reaction of AlkenesSynthesis of Alkenes

Addition reaction

Oxidation reaction

Synthesis

Dehydration of alcohol

Hydrogenation of Al kyne

Dehydrohalogenation of alkyl

halides



Alkenes Synthesis:I: Dehydration of Alcohol It is one of the best method to synthesize of alkenes.

Dehydration=> removal of water, H2O.

This reaction is reversible.

Reagent involved:

Concentrated H2SO4/ H3PO4(as acidic catalyst & dehydrating agent) at

200C or

Al2O3 at high T

General equation:

C CC C

H OH

acid+ HOH



Example

Saytzeff Rule: The major product of alkenes is the comp. with the highest no. of alkyl

substitution on C atom of the double bond.

(c) H2SO4

OH

R2C=CR2> R2C=CHR > RHC=CHR and R2C=CH2> RHC=CH2

2-butanolCH3CH2CH(OH)CH3

H+

H+

2-butene (major product)

1-butene (minor product)



Mechanism of Dehydration Involved formation of carbocation: +ve charged intermediate which is very reactive &

unstable.

+

slow

++ H2O

CH3C CH

3

CH3

OH2

CH3C CH

3

CH3

Step 2: Carbocation formation: The electronegative O atom attract the electron from C, which cause

breaking of a bond. 1 molecule of water released and a carbocation form.

Step 1: Protonation of alcohol: Proton of acid act as catalyst which protonates the OH group into H2O.



+

+ H+

CH3C CH

3

CH3

C CH2

CH3

CH3

Step 3: Removal of 1 proton of carbocation to regenerate the

acid catalyst and form alkenes


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There are 3 type of carbocation: Primary, secondary & tertiary

- The stability & easiest for carbocation forming is 3> 2 > 1

- The tertiary alcohol undergo dehydration easier compared secondary & primary evenin low acidic catalyst.


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WK 3 EP101 / EG101 32School of Engineering32

R C C R'

NH3 or RNH2

R

C C

R'

R

C C

R'

H2 Ni

Li or Na

H2C C

LindlarCatalyst H

C C

H

Alkenes Synthesis:II: Hydrogenation of Alkynes


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Syn-Hydrogenation of Alkynes: cis-Alkenes

.

Theselective catalytic hydrogenation of alkynes with a "poisoned"catalyst is a syn-additionthat yields cis-alkenes. A second way to carry

out this transformation is with a nickel boride catalyst(Ni2B) sold

commercially as P-2

3-hexyne+ H2CH3CH2C CCH2CH3 Ni2B

(Z)-3-hexene (97%)

C CH

H3CH2C

H

CH2CH3


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.

Alkynes are reduced to trans-alkenes by alkali metals (Li or Na) in

liquid ammonia (-33o

C) or ethylamine (BP 16.6o

C) at lowtemperatures. Because these metals dissolve as they react, these and

other similar reductions are called dissolving metal reductions

3-hexyne

(i) Li, C2H5NH2, -78oC

(ii) NH4Cl (workup)

(E)-3-hexene

CH3

CH2

C CCH2

CH3 C C

H3CH2C

H

H

CH2CH3

Anti-Hydrogenation of Alkynes: trans-Alkenes


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examples:

Saytzev rule: to form a highly substituted alkenes

1. Involved removal of H and X from C atom

2. Rxn is in basic condition

3. Alkyl halide is heated in KOH in alcohol to give alkenes.

Alkenes Synthesis:III: Dehydrohalogenation of Alkyl Halide

2OHKX+ ++ KOH

X

Alcohol


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Reaction of AlkenesOxidation

i. cleavage

Addition

i. Hydrogenation

ii. Halogenation (Cl / Br)

iii. Electrophilic (H-A)

iv. H2SO4

v. H20


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Addition Reaction onto C=C

C C C C

A

+ A-B

B

alkenes reagent Additional product

Addition Reaction: (i) H (Hydrogenation)


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Addition Reaction: (ii) Halogenation (Cl / Br)CCl4

halogen


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Addition Reaction: (iii) Electrophilic (HCl / HBr / HI) Hydrogen halide is a polar compound that consist of H+

H+ less of electron is named as electrophilic.

Double bond, in C=C is with electron.

H+ will attack the electrons in bond.

Carbocation intermediate forms.

Nucleophile adds to the carbocation.

Net result is addition to the double bond. H-AH+ + A-


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It is a rxn of cold concentrated sulfuric acid to alkenes and stirred to products alkyl hydrogen

sulfate.

Addition Reaction: (iv) H2SO4

C C+ H

+C

H

C +

C

H

C+ + X:_

C

H

C

X

HOSO3-

HOSO3


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Addition Reaction: (v) H2O


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Both the pi and sigma bonds break.

C=C becomes C=O.

Two methods:

Warm or concentrated or acidic KMnO4.

Ozonolysis

Used to determine the position of a double bond in an unknown.

Oxidation of alkenes: (i) Cleavage

Permanganate is a strong oxidizing agent.

Glycol initially formed is further oxidized.

Disubstituted carbons become ketones. Monosubstituted carbons become carboxylic acids.

Terminal =CH2becomes CO2.

a) Cleavage with MnO4-


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Cleavage by KMnO4

CCCH3 CH3

H CH3 KMnO4

(warm, conc.)C C

CH3

CH3

OHOH

H3C

H

C

O

H3C

H

C

CH3

CH3

O

C

O

H3COH

+


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C

C

CH2CH3

H CH2CH3

C

C

CH2CH3

CH2CH3

OH

OH

H

HH2O2

H

(2)

(1)OsO4

cis-3-hexene meso-3,4-hexanedi

Cleavage by Osmium tetroxide (OsO4)


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b) Ozonolysis Reaction with ozone forms an ozonide. Ozonides are not isolated, but are treated with a mild reducing agent like Zn or dimethyl

sulfide.

Milder oxidation than permanganate.

Products formed are ketones or aldehydes.

CCCH3 CH3

H CH3 O3C

H3C

H

O O

CCH3

CH3

O

Ozonide

+

(CH3)2SC

H3C

HO C

CH3

CH3

O CH3 S

O

CH3

DMSO


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Test for Presence of Alkenes1. Decoloration Test

2. Permanganate Test


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alkanesname contains

CnH(2n+2)

undergo cracking to produce

-ane

petroleum

aqueous

bromine

are

alcohols

substitution

reactions

CO2and H2O

fuels

areusedas

saturated

- CH2-

unsaturated

C=C

CnH2n

-ene

additional

reactions

alcohols alkanes

dibromoalkanes

react with hydrogen to produce

reactwithsteam

toproduce

name contains

are

decolourises

withbrominetoproduce

each member ofhomologous series

differs by

alkenes

Documents

EP101 Sen Lnt 003 Alkene May11