Lecture 7 - Alkenes - Alkenes and Alkynes Notes | EduRev

Created by: Stuti Nagpal

: Lecture 7 - Alkenes - Alkenes and Alkynes Notes | EduRev

 Page 1


NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 1 of 39 
Module 4  Alkenes and Alkynes 
Lecture 7 Alkenes 
4.1 Introduction 
Hydrocarbons that contain carbon-carbon double bond are called Alkenes (also called as 
Olefins). Many alkenes are found in plant and animals. It has three sp
2
 orbitals that lie in 
a plane with angles of 120°. One of the carbon–carbon bonds in a double bond is s-bond, 
formed by the overlap of a sp
2 
orbital of one carbon with a sp
2
 orbital of the other carbon. 
The second carbon–carbon bond in the double bond is formed from side-to-side overlap 
of the remaining p-orbitals of the carbons. These two p-orbitals must be parallel to each 
other to achieve maximum orbital-orbital overlap. Therefore, all six atoms of the double-
bond system are in the same plane (Figure 1). Since there is maximum side-to-side 
overlap, rotation about a double bond does not occur.  
C C
H H
H H
 
Figure 1 
Each C-H s-bond is formed by overlap of a sp
2
 hybrid orbital of carbon atom with the 1s 
orbital of a hydrogen atom. The C-H bond length in ethylene is slightly shorter than the 
C-H bond in ethane because the sp
2
 orbital in ethylene has more s character that attracts 
the electrons even more strongly. The C=C bond in ethylene is much shorter than the C-C 
bond in ethane, partly because the s-bond of ethylene is formed from sp
2
 orbitals and 
partly because both s- and p-bonds are attracting the atoms together (Figure 2).  
1.54 
Å
1.
33
 
Å
1.
09
 
Å
1.
08
 
Å
H
H
H
H
H
H
H H
H H
ethane ethene
 
Figure 2 
Page 2


NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 1 of 39 
Module 4  Alkenes and Alkynes 
Lecture 7 Alkenes 
4.1 Introduction 
Hydrocarbons that contain carbon-carbon double bond are called Alkenes (also called as 
Olefins). Many alkenes are found in plant and animals. It has three sp
2
 orbitals that lie in 
a plane with angles of 120°. One of the carbon–carbon bonds in a double bond is s-bond, 
formed by the overlap of a sp
2 
orbital of one carbon with a sp
2
 orbital of the other carbon. 
The second carbon–carbon bond in the double bond is formed from side-to-side overlap 
of the remaining p-orbitals of the carbons. These two p-orbitals must be parallel to each 
other to achieve maximum orbital-orbital overlap. Therefore, all six atoms of the double-
bond system are in the same plane (Figure 1). Since there is maximum side-to-side 
overlap, rotation about a double bond does not occur.  
C C
H H
H H
 
Figure 1 
Each C-H s-bond is formed by overlap of a sp
2
 hybrid orbital of carbon atom with the 1s 
orbital of a hydrogen atom. The C-H bond length in ethylene is slightly shorter than the 
C-H bond in ethane because the sp
2
 orbital in ethylene has more s character that attracts 
the electrons even more strongly. The C=C bond in ethylene is much shorter than the C-C 
bond in ethane, partly because the s-bond of ethylene is formed from sp
2
 orbitals and 
partly because both s- and p-bonds are attracting the atoms together (Figure 2).  
1.54 
Å
1.
33
 
Å
1.
09
 
Å
1.
08
 
Å
H
H
H
H
H
H
H H
H H
ethane ethene
 
Figure 2 
NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 2 of 39 
Alkenes are said to be unsaturated because they are capable of adding hydrogen in the 
presence of a catalyst. An alkane is called as saturated because it cannot react with any 
more hydrogen. 
4.2 Nomenclature of Alkenes 
• Find the longest continuous chain of carbon atoms that includes the double bond 
and change the -ane ending of the parent alkane to -ene. The chain is numbered as 
the double bond having the lower possible numbers. 
6
5
4
3
2
1
2
3
4
5
6
2-hexene
1
2-
p
ro
y
lhexene
 
• If a chain has more than one substituent, the substituents are cited in alphabetical 
order. The prefixes di, tri, sec, and tert are not considered in alphabetizing, but 
iso, neo, and cyclo are considered. It should also contain the lowest substituent 
number. 
8
7
6
3
2
1 5
4
2,5-
dimeth
y
l-4-octene
 
• The double bond should be inbetween carbon 1 and 2, while numbering the ring. 
5 4
3
2
1
1
6
5
4
3
2
CH
3
CH
3
CH
3
3
-eth
y
lc
y
clo
p
entene
4,5-
dimeth
y
lc
y
clohexene
5
6
1
2
3
4
3
-meth
y
lenec
y
clohexene
 
 
• Lack of rotation of carbon-carbon double bond gives rise to cis-trans isomerism, 
also called geometrical isomerism. In cis-isomer, two similar groups bonded on 
the same side of the double bond. If the similar groups are on opposite sides of the 
double bond, then the alkene is a trans-isomer. 
H
H
3
C CH
2
CH
3
H H
H
3
C H
CH
2
CH
3
cis-2-
p
entene trans-2-
p
entene
 
Page 3


NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 1 of 39 
Module 4  Alkenes and Alkynes 
Lecture 7 Alkenes 
4.1 Introduction 
Hydrocarbons that contain carbon-carbon double bond are called Alkenes (also called as 
Olefins). Many alkenes are found in plant and animals. It has three sp
2
 orbitals that lie in 
a plane with angles of 120°. One of the carbon–carbon bonds in a double bond is s-bond, 
formed by the overlap of a sp
2 
orbital of one carbon with a sp
2
 orbital of the other carbon. 
The second carbon–carbon bond in the double bond is formed from side-to-side overlap 
of the remaining p-orbitals of the carbons. These two p-orbitals must be parallel to each 
other to achieve maximum orbital-orbital overlap. Therefore, all six atoms of the double-
bond system are in the same plane (Figure 1). Since there is maximum side-to-side 
overlap, rotation about a double bond does not occur.  
C C
H H
H H
 
Figure 1 
Each C-H s-bond is formed by overlap of a sp
2
 hybrid orbital of carbon atom with the 1s 
orbital of a hydrogen atom. The C-H bond length in ethylene is slightly shorter than the 
C-H bond in ethane because the sp
2
 orbital in ethylene has more s character that attracts 
the electrons even more strongly. The C=C bond in ethylene is much shorter than the C-C 
bond in ethane, partly because the s-bond of ethylene is formed from sp
2
 orbitals and 
partly because both s- and p-bonds are attracting the atoms together (Figure 2).  
1.54 
Å
1.
33
 
Å
1.
09
 
Å
1.
08
 
Å
H
H
H
H
H
H
H H
H H
ethane ethene
 
Figure 2 
NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 2 of 39 
Alkenes are said to be unsaturated because they are capable of adding hydrogen in the 
presence of a catalyst. An alkane is called as saturated because it cannot react with any 
more hydrogen. 
4.2 Nomenclature of Alkenes 
• Find the longest continuous chain of carbon atoms that includes the double bond 
and change the -ane ending of the parent alkane to -ene. The chain is numbered as 
the double bond having the lower possible numbers. 
6
5
4
3
2
1
2
3
4
5
6
2-hexene
1
2-
p
ro
y
lhexene
 
• If a chain has more than one substituent, the substituents are cited in alphabetical 
order. The prefixes di, tri, sec, and tert are not considered in alphabetizing, but 
iso, neo, and cyclo are considered. It should also contain the lowest substituent 
number. 
8
7
6
3
2
1 5
4
2,5-
dimeth
y
l-4-octene
 
• The double bond should be inbetween carbon 1 and 2, while numbering the ring. 
5 4
3
2
1
1
6
5
4
3
2
CH
3
CH
3
CH
3
3
-eth
y
lc
y
clo
p
entene
4,5-
dimeth
y
lc
y
clohexene
5
6
1
2
3
4
3
-meth
y
lenec
y
clohexene
 
 
• Lack of rotation of carbon-carbon double bond gives rise to cis-trans isomerism, 
also called geometrical isomerism. In cis-isomer, two similar groups bonded on 
the same side of the double bond. If the similar groups are on opposite sides of the 
double bond, then the alkene is a trans-isomer. 
H
H
3
C CH
2
CH
3
H H
H
3
C H
CH
2
CH
3
cis-2-
p
entene trans-2-
p
entene
 
NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 3 of 39 
 
• Cycloalkenes are preferred to be cis unless the ring is large enough (at least eight 
carbon atoms) to be trans. 
 
cis-c
y
clodecene c
y
clohexene c
y
clooctene
trans-c
y
clodecene
 
 
• Cis-trans nomenclature cannot be used for the alkenes having four different 
groups. For example, 1-bromo-1-chloropropene is not clearly cis or trans as there 
is no similar groups. Such alkene can be named using E-Z Nomenclature. 
 
Cl
Br
CH
3
H Br
Cl CH
3
H
1-
b
romo-1-chloro
p
ro
p
ene
 
• Each end of the double bond should be considered separately. Assign first and 
second priorities to the two substituent groups on one end of the double bond. Do 
the same for the other end. If the two first-priority atoms are on the same side of 
the double bond, it is called as Z isomer. If the two first-priority atoms are on 
opposite sides of the double bond, it is then called as E isomer. 
 
Cl
Br
CH
3
H Br
Cl CH
3
H
1 1
2 2
1
2
2
1
(
Z
)
-1-
b
romo-1-chloro
p
ro
p
ene
(
E
)
-1-
b
romo-1-chloro
p
ro
p
ene
 
 
Br
(
3Z,5E
)
-
3
-
b
romoocta-
3,5-
diene
O CH
3
(
E
)
-isomer
 
 
 
 
Page 4


NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 1 of 39 
Module 4  Alkenes and Alkynes 
Lecture 7 Alkenes 
4.1 Introduction 
Hydrocarbons that contain carbon-carbon double bond are called Alkenes (also called as 
Olefins). Many alkenes are found in plant and animals. It has three sp
2
 orbitals that lie in 
a plane with angles of 120°. One of the carbon–carbon bonds in a double bond is s-bond, 
formed by the overlap of a sp
2 
orbital of one carbon with a sp
2
 orbital of the other carbon. 
The second carbon–carbon bond in the double bond is formed from side-to-side overlap 
of the remaining p-orbitals of the carbons. These two p-orbitals must be parallel to each 
other to achieve maximum orbital-orbital overlap. Therefore, all six atoms of the double-
bond system are in the same plane (Figure 1). Since there is maximum side-to-side 
overlap, rotation about a double bond does not occur.  
C C
H H
H H
 
Figure 1 
Each C-H s-bond is formed by overlap of a sp
2
 hybrid orbital of carbon atom with the 1s 
orbital of a hydrogen atom. The C-H bond length in ethylene is slightly shorter than the 
C-H bond in ethane because the sp
2
 orbital in ethylene has more s character that attracts 
the electrons even more strongly. The C=C bond in ethylene is much shorter than the C-C 
bond in ethane, partly because the s-bond of ethylene is formed from sp
2
 orbitals and 
partly because both s- and p-bonds are attracting the atoms together (Figure 2).  
1.54 
Å
1.
33
 
Å
1.
09
 
Å
1.
08
 
Å
H
H
H
H
H
H
H H
H H
ethane ethene
 
Figure 2 
NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 2 of 39 
Alkenes are said to be unsaturated because they are capable of adding hydrogen in the 
presence of a catalyst. An alkane is called as saturated because it cannot react with any 
more hydrogen. 
4.2 Nomenclature of Alkenes 
• Find the longest continuous chain of carbon atoms that includes the double bond 
and change the -ane ending of the parent alkane to -ene. The chain is numbered as 
the double bond having the lower possible numbers. 
6
5
4
3
2
1
2
3
4
5
6
2-hexene
1
2-
p
ro
y
lhexene
 
• If a chain has more than one substituent, the substituents are cited in alphabetical 
order. The prefixes di, tri, sec, and tert are not considered in alphabetizing, but 
iso, neo, and cyclo are considered. It should also contain the lowest substituent 
number. 
8
7
6
3
2
1 5
4
2,5-
dimeth
y
l-4-octene
 
• The double bond should be inbetween carbon 1 and 2, while numbering the ring. 
5 4
3
2
1
1
6
5
4
3
2
CH
3
CH
3
CH
3
3
-eth
y
lc
y
clo
p
entene
4,5-
dimeth
y
lc
y
clohexene
5
6
1
2
3
4
3
-meth
y
lenec
y
clohexene
 
 
• Lack of rotation of carbon-carbon double bond gives rise to cis-trans isomerism, 
also called geometrical isomerism. In cis-isomer, two similar groups bonded on 
the same side of the double bond. If the similar groups are on opposite sides of the 
double bond, then the alkene is a trans-isomer. 
H
H
3
C CH
2
CH
3
H H
H
3
C H
CH
2
CH
3
cis-2-
p
entene trans-2-
p
entene
 
NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 3 of 39 
 
• Cycloalkenes are preferred to be cis unless the ring is large enough (at least eight 
carbon atoms) to be trans. 
 
cis-c
y
clodecene c
y
clohexene c
y
clooctene
trans-c
y
clodecene
 
 
• Cis-trans nomenclature cannot be used for the alkenes having four different 
groups. For example, 1-bromo-1-chloropropene is not clearly cis or trans as there 
is no similar groups. Such alkene can be named using E-Z Nomenclature. 
 
Cl
Br
CH
3
H Br
Cl CH
3
H
1-
b
romo-1-chloro
p
ro
p
ene
 
• Each end of the double bond should be considered separately. Assign first and 
second priorities to the two substituent groups on one end of the double bond. Do 
the same for the other end. If the two first-priority atoms are on the same side of 
the double bond, it is called as Z isomer. If the two first-priority atoms are on 
opposite sides of the double bond, it is then called as E isomer. 
 
Cl
Br
CH
3
H Br
Cl CH
3
H
1 1
2 2
1
2
2
1
(
Z
)
-1-
b
romo-1-chloro
p
ro
p
ene
(
E
)
-1-
b
romo-1-chloro
p
ro
p
ene
 
 
Br
(
3Z,5E
)
-
3
-
b
romoocta-
3,5-
diene
O CH
3
(
E
)
-isomer
 
 
 
 
NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 4 of 39 
4.3 Stability of Alkenes 
The stability of an alkene depends on its structure. The heat released in a hydrogenation 
reaction is called the heat of hydrogenation. When an alkene is treated with hydrogen in 
the presence of a platinum catalyst, hydrogen adds to the double bond, reducing the 
alkene to an alkane. Hydrogenation is exothermic, evolving about 20 to 30 kcal of heat 
per mole of hydrogen consumed.  
The difference in the stabilities of alkenes is the difference in their heats of 
hydrogenation. While considering the hydrogenation of 1-butene (a monosubstituted 
alkene), 2-butene (a disubstituted alkene) and 2-methyl-2-butene (a trisubstituted alkene), 
2-methyl-2-butene is more stable by 3.4 kcal/mol and 2-butene is stable by 2.7 kcal/mol 
(Scheme 1). More substituted double bonds are usually more stable. In other words, the 
alkyl groups attached to the double bonded carbons stabilize the alkene. 
H
3
C
H
3
C
CH
3
CH
3
H
3
C
CH
3
H
3
C
(
monosubstituted
 alkene
)
(
disubstituted
 alkene
)
H
2
Pt
H
2
Pt
?
H°
 = -
30
.
3
 kcal/
mol
?
H°
 = -27.
6
 kcal/
mol
?
H°
 = 
30
.
3
 kcal/
mol - 27.
6
 kcal/
mol = 2.7 kcal/
mol
1-
butene
2-
butene
butane
butane
H
3
C
CH
3
CH
3
H
3
C
(
trisubstituted
 alkene
)
H
2
Pt
?
H°
 = -26
.
9
 kcal/
mol
CH
3
2-meth
y
l-2-
butene
2-meth
y
l-2-
butane
CH
3
?
H°
 = 
30
.
3
 kcal/
mol - 26
.
9
 kcal/
mol = 
3
.4 kcal/
mol
 
Scheme 1 
 
 
Page 5


NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 1 of 39 
Module 4  Alkenes and Alkynes 
Lecture 7 Alkenes 
4.1 Introduction 
Hydrocarbons that contain carbon-carbon double bond are called Alkenes (also called as 
Olefins). Many alkenes are found in plant and animals. It has three sp
2
 orbitals that lie in 
a plane with angles of 120°. One of the carbon–carbon bonds in a double bond is s-bond, 
formed by the overlap of a sp
2 
orbital of one carbon with a sp
2
 orbital of the other carbon. 
The second carbon–carbon bond in the double bond is formed from side-to-side overlap 
of the remaining p-orbitals of the carbons. These two p-orbitals must be parallel to each 
other to achieve maximum orbital-orbital overlap. Therefore, all six atoms of the double-
bond system are in the same plane (Figure 1). Since there is maximum side-to-side 
overlap, rotation about a double bond does not occur.  
C C
H H
H H
 
Figure 1 
Each C-H s-bond is formed by overlap of a sp
2
 hybrid orbital of carbon atom with the 1s 
orbital of a hydrogen atom. The C-H bond length in ethylene is slightly shorter than the 
C-H bond in ethane because the sp
2
 orbital in ethylene has more s character that attracts 
the electrons even more strongly. The C=C bond in ethylene is much shorter than the C-C 
bond in ethane, partly because the s-bond of ethylene is formed from sp
2
 orbitals and 
partly because both s- and p-bonds are attracting the atoms together (Figure 2).  
1.54 
Å
1.
33
 
Å
1.
09
 
Å
1.
08
 
Å
H
H
H
H
H
H
H H
H H
ethane ethene
 
Figure 2 
NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 2 of 39 
Alkenes are said to be unsaturated because they are capable of adding hydrogen in the 
presence of a catalyst. An alkane is called as saturated because it cannot react with any 
more hydrogen. 
4.2 Nomenclature of Alkenes 
• Find the longest continuous chain of carbon atoms that includes the double bond 
and change the -ane ending of the parent alkane to -ene. The chain is numbered as 
the double bond having the lower possible numbers. 
6
5
4
3
2
1
2
3
4
5
6
2-hexene
1
2-
p
ro
y
lhexene
 
• If a chain has more than one substituent, the substituents are cited in alphabetical 
order. The prefixes di, tri, sec, and tert are not considered in alphabetizing, but 
iso, neo, and cyclo are considered. It should also contain the lowest substituent 
number. 
8
7
6
3
2
1 5
4
2,5-
dimeth
y
l-4-octene
 
• The double bond should be inbetween carbon 1 and 2, while numbering the ring. 
5 4
3
2
1
1
6
5
4
3
2
CH
3
CH
3
CH
3
3
-eth
y
lc
y
clo
p
entene
4,5-
dimeth
y
lc
y
clohexene
5
6
1
2
3
4
3
-meth
y
lenec
y
clohexene
 
 
• Lack of rotation of carbon-carbon double bond gives rise to cis-trans isomerism, 
also called geometrical isomerism. In cis-isomer, two similar groups bonded on 
the same side of the double bond. If the similar groups are on opposite sides of the 
double bond, then the alkene is a trans-isomer. 
H
H
3
C CH
2
CH
3
H H
H
3
C H
CH
2
CH
3
cis-2-
p
entene trans-2-
p
entene
 
NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 3 of 39 
 
• Cycloalkenes are preferred to be cis unless the ring is large enough (at least eight 
carbon atoms) to be trans. 
 
cis-c
y
clodecene c
y
clohexene c
y
clooctene
trans-c
y
clodecene
 
 
• Cis-trans nomenclature cannot be used for the alkenes having four different 
groups. For example, 1-bromo-1-chloropropene is not clearly cis or trans as there 
is no similar groups. Such alkene can be named using E-Z Nomenclature. 
 
Cl
Br
CH
3
H Br
Cl CH
3
H
1-
b
romo-1-chloro
p
ro
p
ene
 
• Each end of the double bond should be considered separately. Assign first and 
second priorities to the two substituent groups on one end of the double bond. Do 
the same for the other end. If the two first-priority atoms are on the same side of 
the double bond, it is called as Z isomer. If the two first-priority atoms are on 
opposite sides of the double bond, it is then called as E isomer. 
 
Cl
Br
CH
3
H Br
Cl CH
3
H
1 1
2 2
1
2
2
1
(
Z
)
-1-
b
romo-1-chloro
p
ro
p
ene
(
E
)
-1-
b
romo-1-chloro
p
ro
p
ene
 
 
Br
(
3Z,5E
)
-
3
-
b
romoocta-
3,5-
diene
O CH
3
(
E
)
-isomer
 
 
 
 
NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 4 of 39 
4.3 Stability of Alkenes 
The stability of an alkene depends on its structure. The heat released in a hydrogenation 
reaction is called the heat of hydrogenation. When an alkene is treated with hydrogen in 
the presence of a platinum catalyst, hydrogen adds to the double bond, reducing the 
alkene to an alkane. Hydrogenation is exothermic, evolving about 20 to 30 kcal of heat 
per mole of hydrogen consumed.  
The difference in the stabilities of alkenes is the difference in their heats of 
hydrogenation. While considering the hydrogenation of 1-butene (a monosubstituted 
alkene), 2-butene (a disubstituted alkene) and 2-methyl-2-butene (a trisubstituted alkene), 
2-methyl-2-butene is more stable by 3.4 kcal/mol and 2-butene is stable by 2.7 kcal/mol 
(Scheme 1). More substituted double bonds are usually more stable. In other words, the 
alkyl groups attached to the double bonded carbons stabilize the alkene. 
H
3
C
H
3
C
CH
3
CH
3
H
3
C
CH
3
H
3
C
(
monosubstituted
 alkene
)
(
disubstituted
 alkene
)
H
2
Pt
H
2
Pt
?
H°
 = -
30
.
3
 kcal/
mol
?
H°
 = -27.
6
 kcal/
mol
?
H°
 = 
30
.
3
 kcal/
mol - 27.
6
 kcal/
mol = 2.7 kcal/
mol
1-
butene
2-
butene
butane
butane
H
3
C
CH
3
CH
3
H
3
C
(
trisubstituted
 alkene
)
H
2
Pt
?
H°
 = -26
.
9
 kcal/
mol
CH
3
2-meth
y
l-2-
butene
2-meth
y
l-2-
butane
CH
3
?
H°
 = 
30
.
3
 kcal/
mol - 26
.
9
 kcal/
mol = 
3
.4 kcal/
mol
 
Scheme 1 
 
 
NPTEL – Biotechnology – Cell Biology 
 
 
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Alkene, which releases the most heat, must be the least stable. In contrast, the alkene, 
which releases the least heat, must be the most stable.  
C C
R
R R
R
C C
R
H R
R
C C
H
H R
R
C C
H
H H
R
relative stabilities of
 alk
y
l-substituted
 alkenes
more
stable
least
stable
> > >
 
The heats of hydrogenation show that trans-isomers are generally more stable than the 
corresponding cis-isomers. Because the alkyl substituents are separated farther in trans-
isomers than they are in cis-isomers. The greater stability of the trans-isomer is evident in 
the following example, which shows that the trans-isomer is stable by 1.0 kcal/mol 
(Scheme 2). 
?
H°
 = -28
.
6
 kcal/
mol
?
H°
 = -27.
6
 kcal/
mol
?
H°
 = 28
.
6
 kcal/
mol - 27.
6
 kcal/
mol = 1.
0
 kcal/
mol
H
3
C CH
2
CH
3
H H
H
3
C H
H CH
2
CH
3
2-
p
entene
(
cis-alkene
)
(
trans-alkene
)
2-
p
entene
H
2
Pt
H
2
Pt
H
3
C CH
3
H
3
C CH
3
p
entane
p
entane
 
Scheme 2 
 
 
 
 
 
 
 
 
 
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