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Alkenes
• Alkene:Alkene: a hydrocarbon that contains one or more carbon-carbon double bonds– Ethylene (Ethene) is the simplest alkene
Alkynes
• Alkyne:Alkyne: a hydrocarbon that contains one or more carbon-carbon triple bonds– Acetylene (Ethyne) is the simplest alkyne
Alkenes
• Structure:– VSEPR model predicts bond angles of 120°– In substituted alkenes, may vary due to the repulsion of
alkyl groups
H
C C
H
H H
121.7°H3C
C C
H
H H
124.7°
Ethylene Propene
Alkenes - IUPAC Names
• To name an alkene– the parent name is that of the longest chain that contains
the C=C– number the chain from the end that gives the lower
numbers to the carbons of the C=C– locate the C=C by its first carbon– use the ending -eneene– branched-chain alkenes are named in a manner similar to
alkanes; substituted groups are numbered and named
Alkenes - IUPAC Names
• Examples
CH3CH2CH2CH2CH=CH2 CH3CH2CHCH2CH=CH2
CH3
CH3CH2CHC=CH2CH2CH3
CH2CH3
1-Hexene 4-Methyl-1-hexene 2,3-Diethyl-1-pentene
1 1
1
2 22
3 3
3
4 44
5 5
5
6 6
Alkynes - IUPAC Names
– follow the same rules as for alkenes, but use the ending -yneyne
CH3CHC CHCH3
CH3CH2C CCH2CCH3
CH3
CH3
3-Methyl-1-butyne 6,6-Dimethyl-3-heptyne
1 1
2 23
3 44 5
6 7
Common Names
• Common names are still used for some
CH3CH=CH2 CH3C=CH2
CH3
CH2=CH2
IsobutylenePropyleneCommon name:IUPAC name: 2-MethylpropenePropene
EthyleneEthene
CH3C CH CH3C CCH3HC CH
Common name:IUPAC name:
Methylacetylene DimethylacetylenePropyne 2-ButyneEthyne
Acetylene
Cycloalkenes
• To name a cycloalkene– number the carbon atoms of the ring double bond 1 and 2
in the direction that gives the lower number to the substituent encountered first
– list substituents in alphabetical order
1 2
3
4
5
1
2
3
4
5
6
3-Methylcyclopentene(not 5-methylcyclopentene)
4-Ethyl-1-methylcyclohexene(not 5-ethyl-2-methylcyclohexene)
Dienes, Trienes, Polyenes– di, tri, tetra … in front of “ene”.
CH2=CCH=CH2
CH3
CH2=CHCH2CH=CH21,4-Pentadiene 2-Methyl-1,3-butadiene
(Isoprene)1,3-Cyclopentadiene
Alkenes
• Cis-trans isomerism– because of restricted rotation, an alkene with two
different groups on each carbon of the double bond shows cis-trans isomerism
trans-2-Butenemp -106°C, bp 1°C
cis-2-Butenemp -139°C, bp 4°C
HC C
CH3
H3C H
HC C
H
H3C CH3
Alkenes
Alkenes – Naming
CH2 CH CH2 CH3
1-butene
CH3 CH CH CH3
2-butene
C C
Cl Cl
H H
C C
Cl H
H Cl
cis-1,2-dichloroethene trans-1,2-dichloroethene
The E,Z System• Alkenes with four different substituents cannot
be named cis or trans without some sort of rules. A Newer naming scheme – E/Z is now IUPAC
The E,Z System
• the E,Z system is a way to distinguish between highly substituted alkenes in a systematic way. The cis trans designation is not used in the chemistry world much any more.
The E,Z System
• The first step in assigning an E or Z configuration is to determine each groups priority– priority is based on atomic number– the higher the atomic number, the higher the
priority– Go from point of difference
The E,Z System-Priority
– Example:Example: assign priorities to the groups in each set
-CH2OH -CH2CH2OH-CH2CH2OH -CH2NH2(a) (b)and and
-CH2OH -CH2CH2COH
O
-CH2NH2 -CH2COH
O
and(c) (d)and
Physical Properties
– alkenes and alkynes are nonpolar compounds– the only attractive forces between their molecules are
London dispersion forces– their physical properties are similar to those of alkanes
with the same carbon skeletons– alkenes and alkynes are insoluble in water but soluble in
one another and in nonpolar organic liquids – alkenes and alkynes that are liquid or solid at room
temperature have densities less than 1 g/mL; they float on water
Addition Reactions
• Alkenes undergo reactions known as addition reactions because atoms are added “across” the double bond.
Reactions of Alkenes• The most common reaction is addition
CC
C C
C C Br2
HCl
H2O
CC H2
C CBr Br
C CH Cl
C CH OH
C CHH
Descriptive Name(s )Reaction
+
+
+
+
bromination
hydrochlorination
hydration
hydrogenation(reduction)
Reactions of Alkenes• Most alkene addition reactions are exothermic
– the products are more stable (lower in energy) than the reactants
– exothermic doesn’t mean rapid– reaction rate depends on activation energy– a catalyst is often required
H
C C
H
HH
+ H C C H
H
H
H
H
H H
one double bondand one single bond
three single bonds
are replaced by+ heat
Addition of HX• Addition of HX (HCl, HBr, or HI) to an alkene gives a
haloalkane– H adds to one carbon and X to the other
– reaction is regioselectiveregioselective– Markovnikov’s rule:Markovnikov’s rule: H adds to the less substituted carbon
and X to the more substituted carbon
CH2=CH2 HCl CH2-CH2
ClH
Chloroethane(Ethyl chloride)
Ethylene
+
CH3CH=CH2 HCl CH3CH-CH2
HClCH3CH-CH2
ClH
1-Chloropropane(not formed)
2-ChloropropanePropene
+
Addition of H2O• Addition of water is called hydrationhydration
– hydration is acid catalyzed, usually by H2SO4
– hydration follows Markovnikov’s rule; H adds to the less substituted carbon and OH adds to the more substituted carbon
CH3CH=CH2 H2OH2SO4
CH3CH-CH2
HOH
Propene 2-Propanol+
CH3C=CH2
CH3
H2OH2SO4 CH3C-CH2
CH3
HOH2-Methyl-2-propanol2-Methylpropene
+
Addition of Cl2 and Br2
• Addition takes place readily at room temperature– reaction is generally carried out using pure reagents, or mixing them
in a nonreactive organic solvent
– addition of Br2 is a useful qualitative test for the presence of a carbon-carbon double bond
– Br2 has a deep red color; dibromoalkanes are colorless
Br2 CH2Cl2
Br
Br+
1,2-DibromocyclohexaneCyclohexene
CH3CH=CHCH3 Br2 CH2Cl2CH3CH-CHCH3
Br Br
2,3-Dibromobutane2-Butene
+
Addition of H2 - Reduction
• Virtually all alkenes add H2 in the presence of a transition metal catalyst, commonly Pd, Pt, or Ni
HH3C
C C
H CH3
PdCH3CH2CH2CH3
trans-2-Butene
+ H225°C, 3 atm
Butane
Pd+ H2
Cyclohexene Cyclohexane
25°C, 3 atm
24.2
Alkynes
Alkynes – Name longest chain which contains the triple bond and end with “yne.” Number the location.
• contain at least one carbon-carbon triple bond
1-butyne 2-butyne
CH C CH2 CH3 CH3 C C CH3
24.2
Alkyne Reactions
Addition Reactions (also difficult to stop)
Hydrogenation
CH CH (g) + H2 (g) CH2 CH2 (g)
CH CH (g) + HBr (g) CH2 CHBr (g)
CH CH (g) + Br2 (g) CHBr CHBr (g)
CH CH (g) + 2Br2 (g) CHBr2 CHBr2 (g)
Hard to stop at alkene … ends in alkane if not careful.
Alkene Reactions
Production of alkenes by Cracking
Addition Reactions
CH2 CH2 (g) + HBr (g) CH3 CH2Br (g)
CH2 CH2 (g) + Br2 (g) CH2Br CH2Br (g)
C2H6 (g) CH2 CH2 (g) + H2 (g)Pt
catalyst
Addition of HX
• Chemists account for the addition of HX to an alkene by a two-step reaction mechanismreaction mechanism– we use curved arrows to show the repositioning of
electron pairs during a chemical reaction– the tail of an arrow shows the origin of the electron pair
(either on an atom of in a bond)– the head of the arrow shows its new position– curved arrows show us which bonds break and which new
ones form
Addition of HCl to 2 Butene
• Step 1: – the double bond attacks H+ giving a secondary carbocation
intermediate
• Step 2:– reaction of the carbocation intermediate with chloride ion
completes the addition
Cl CH3CHCH2CH3
Cl
CH3CHCH2CH3Chloride
ion2° Carbocationintermediate
- ++
2-Chlorobutane
::::
:
::
CH3CH=CHCH3 H+ CH3CH-CHCH3
H+
A 2° carbocation intermediate
+
Questions to Ponder
+
What do you think the geometry of the carbon that is + charged would be? Why?
Considering the intermediate, what do you expect the product ratios to be? Why?
Br
Addition of H2O
• Step 1:
• Step 2:
• Step 3:
CH3CH=CH2 H+ CH3CHCH2
H+
A 2° carbocationintermediate
+
CH3CHCH3 O-HH
CH3CHCH3
OHH
+ ++
An oxonium ion
:
:
:
CH3CHCH3
OHH
CH3CHCH3
OHH++
+
: :
:
2-Propanol
Polymerization
• From the perspective of the organic chemical industry, the single most important reaction of alkenes is polymerization– polymer:polymer: Greek: poly, many and meros, part– monomer:monomer: Greek: mono, single and meros, part
nCH2=CH2 CH2CH2initiator
Ethylene Polyethylenen(polymerization)