Jones M., Fleming S.A. Organic Chemistry

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3.7 Double Bonds in Rings 119

WORKED PROBLEM 3.18 Draw (E)- and (Z)-1-methylcycloheptene. Which isomer

would you expect to be more stable? Explain.

ANSWER In (Z)-1-methylcycloheptene,the two higher priority groups are on the

same side of the double bond, whereas in (E)-1-methylcycloheptene they are on

opposite sides. On carbon 2, hydrogen is lower priority than CH

, and on carbon 1,

is lower priority than The compound with the higher priority

groups on the same side is (Z), and the compound with the higher priority groups

on opposite sides is (E).

In this example, even the drawings give a clue to the relative stabilities. It is triv-

ial to draw the (Z) form, but the (E) form requires you to stretch bonds (arrow)

in order to make the necessary connections. If you made a model, you saw that

the (E) isomer contains a badly twisted double bond, with poor overlap between

the 2p orbitals making up the π bond.There are no such problems in the (Z) iso-

mer, which is therefore much more stable.

Lower priority

(bottom carbon)

( Z)-1-Methylcycloheptene ( E)-1-Methylcycloheptene

Lower priority

(top carbon)

Lower priority

(top carbon)

Higher priority

(top carbon)

Higher priority

(top carbon)

Higher priority

(bottom carbon)

Higher priority

(bottom carbon)

Note long

bond

Lower priority

(bottom carbon)

All the double bonds in Figures 3.38 and 3.39 are cis. For some reason this

is often a sticky point for students. Make sure you understand why all the dou-

ble bonds shown in the figures are appropriately called cis. To make the point

absolutely clear we will use cyclopentene as an example. Note that the two

hydrogens on the double bond are on the same side. The two alkyl groups (part

of the ring) are on the other side (Fig. 3.40). The molecule is properly called

cis, or (Z).

It is extremely difficult to put a trans double bond in a small ring,and it’s worth

our time to see why. As we saw in Section 3.2, double bonds depend on overlap of

p orbitals for their stability. There is no great problem in a ring containing a cis

double bond (Fig. 3.40), but if the cycloalkene is trans, the double bond becomes

severely twisted and highly destabilized (see Worked Problem 3.18). Remember

that π bonds derive their stability from overlap of 2p orbitals, and that a fully twist-

ed (90°) π bond, with its 2p orbitals oriented at 90° to one another, is about

66 kcal/mol higher in energy than the 0° form (p. 106). For a small or medium-sized

ring, it is not possible to bridge opposite sides of the double bond without very

cis-Cyclopentene

The two hydrogens are on

the same side of the double

bond—the double bond in

this molecule is cis, or (Z )

FIGURE 3.40 The double bond in

cyclopentene is cis.

120 CHAPTER 3 Alkenes and Alkynes

severe strain. Let’s use cyclopentene as an example again. A single CH

group

cannot span the two carbons attached to opposite sides of a trans double bond

(Fig. 3.41).This difficulty is easy to see in models. Use your models to try to make

trans-cyclopentene. Hold the double bond planar, and then try to introduce the

three bridging methylene groups.They won’t reach. Next, allow the π bond to relax

as you connect the methylenes. The methylenes will reach this time, but now the

2p orbitals making up the π bond no longer overlap efficiently. Make sure you see

this point.

To start a trans double bond,

put the two hydrogens on

opposite sides of the

π bond

Now connect the carbons of the double bond

with a chain of methylene (CH

)

groups; if

the ring is large enough (n

–

6), there is no

great problem in forming the trans isomer

To make trans-cyclopentene, the double

bond carbons must be bridged by only

three methylenes; this cannot be done

without twisting the 2p orbitals out of overlap;

this twisting is very costly in energy terms

(CH

)

FIGURE 3.41 It is difficult to

incorporate a trans double bond in a

small ring. Either the bridge of

methylene groups is too small to span

the trans positions or the 2p orbitals

making up the π bond must be

twisted out of overlap.

Of course, the larger the ring, the less problem there is because more atoms

are available to span the trans positions of the double bond. In practice, the

smallest trans cycloalkene stable at room temperature is trans-cyclooctene.

Even here, the trans isomer is 11.4 kcal/mol less stable than its cis counterpart

(Fig. 3.42).

trans-Cyclooctene

WEB 3D

FIGURE 3.42 cis- and trans-Cyclooctene.

PROBLEM 3.19 Calculate the equilibrium distribution of cis- and trans-

cyclooctene at 25 °C given an energy difference of 11.4 kcal/mol between the

two isomers (of course, this question assumes that equilibrium can be reached—

something not generally true for alkenes). The relationship between the energy

difference (ΔG°) and the equilibrium constant (K) is ΔG° RT ln K, or

ΔG° 2.3RT log K, where R is the gas constant (1.986 cal/deg mol) and T

is the absolute temperature.

cis-Cyclooctene

WEB 3D

3.7 Double Bonds in Rings 121

It was noticed in 1924 by the German chemist Julius Bredt (1855–1937) that

one structural type was conspicuously absent among the myriad compounds isolat-

ed from sources in Nature. There seemed to be no double bonds attached to the

bridgehead position in what are called “bridged bicyclic molecules.” The bridge-

head position is the point at which the bridges meet (Fig. 3.44). Apparently, great

Many cyclic and polycyclic (more than one ring) compounds containing double

bonds are known, including a great many natural products. Figure 3.43 shows some

common structures incorporating cycloalkenes.

Vitamin ALimonene

Cholesterol

Morphine

FIGURE 3.43 Some natural products

incorporating cycloalkenes.

Bridged bicyclic system

Bridgehead

position

Bridgehead

position

This double bond

is at the bridge-

head position

Bridgehead

position

Fused bicyclic system

Three shared

carbons

Two shared

adjacent carbons

Bridgehead

position

FIGURE 3.44 Fused and bridged

bicyclic molecules.

instability is caused by the incorporation of a double bond at the bridgehead

position. You have met such structures before in Chapter 2 (p. 85), and they will

reappear in some detail in Chapter 5, but you might make a model now of a simple

bridged bicyclic structure. Figure 3.44 shows two kinds of bicyclic molecules,

“bridged” and “fused.” In fused compounds, two rings share a pair of adjacent car-

bons. In bridged bicyclic compounds, more than two carbons are shared.

122 CHAPTER 3 Alkenes and Alkynes

Bredt was unable to explain this absence of double bonds at the bridgehead

position, but was alert enough to note the phenomenon—the absence of molecules

with double bonds at the bridgehead.That there could not be such compounds has

become deservedly known as Bredt’s rule.What is the reason behind this rule? First

of all, the rigid cage structure with its pyramidal bridgehead carbons (Fig. 3.44)

requires that the π bond, the “double”part of the alkene, be formed not from 2p/2p

overlap, but by 2p/hybrid orbital overlap. Overlap is not as good as in a normal

alkene π system (Fig. 3.45). There is even more to this question, however, and a

PROBLEM 3.20 Is the bicyclo[3.3.1]non-1-ene shown in Figure 3.47 (Z) or (E)?

Bridgehead

This bridgehead carbon cannot become

planar

—flatten out—the rigid cage

prevents this. The orbital on this carbon

cannot be a pure p orbital; it must

be a hybrid, and will therefore not

overlap well with an adjacent p orbital

FIGURE 3.45 Three views of a bridged bicyclic molecule containing a double bond at the bridgehead.

Poor orbital

overlap

FIGURE 3.46 A Newman projection

shows that in these bridgehead

alkenes the orbitals making up the

“π bond” cannot overlap well.

FIGURE 3.47 The smallest bridgehead

alkene stable under normal conditions

is bicyclo[3.3.1]non-1-ene. This

molecule contains a trans-cyclooctene,

shown in color.

careful three-dimensional drawing of the molecule best reveals the story. As is so

often the case, a Newman projection is the most informative way to look at the mol-

ecule (Fig. 3.46). Note that the orbitals making up the (hypothetical) double bond

do not overlap at all well. Just as trans cycloalkenes are severely twisted, so are

bridgehead alkenes. Moreover, the rigid structure of these compounds (make a

model!) allows for no relief. This is a good example of how the two dimensions

of the paper can fool you. There is no difficulty in drawing the lines making up the

double bond on the paper, and unless you can see the structure, you will almost

certainly be fooled.

As the bridges in bicyclic molecules get longer, flexibility returns and bridgehead

“anti-Bredt” alkenes become stable (Fig. 3.47). Clever syntheses have been devised so

that even quite unstable compounds can be made and studied, and old Bredt’s rule

violated. In simple bridgehead alkenes, the limits of room temperature stability are

reached with bicyclo[3.3.1]non-1-ene (do not worry yet about the naming system;

we will deal with it in Chapter 5), which contains a trans double bond in an eight-

membered ring.As with the simple trans cycloalkenes, it is the eight-carbon compound

that is the first molecule stable at room temperature. It doesn’t seem unreasonable that

there should be a rough correspondence between the trans cycloalkenes and the bridge-

head alkenes, which also contain a trans double bond in a ring.

3.9 Alkynes: Structure and Bonding 123

PROBLEM 3.21 Draw the other form of bicyclo[3.3.1]non-1-ene (Z or E).Why is

it less stable than the one in Figure 3.47? Hint: Focus on the rings in which the

double bonds are contained.

TABLE 3.2 Some Simple Alkenes

Name Formula mp (°C) bp (°C)

Ethene (ethylene) 169 103.7

Propene (propylene) 185.2 47.4

1-Butene 185 6.3

cis-2-Butene 138.9 3.7

trans-2-Butene 105.5 0.9

2-Methylpropene (isobutene) 40.7 6.6

1-Pentene 165 30.1

1-Hexene 139.8 63.3

1-Heptene 119 93.6

1-Octene 101.7 121.3

1-Nonene 83 146

1-Decene 66.3 170.5H

(CH

)

(CH

)

(CH

)

(CH

)

(CH

)

(CH

)

(CH

)

TABLE 3.3 Some Simple Cycloalkenes

Name mp (°C) bp (°C)

Cyclobutene 2

Cyclopentene 135 44.2

Cyclohexene 103.5 83

Cycloheptene 56 115

cis-Cyclooctene 12 138

trans-Cyclooctene 59 143

cis-Cyclononene 167–169

trans-Cyclononene 94–96 (at 30 mmHg)

3.8 Physical Properties of Alkenes

There is little difference in physical properties between the alkenes and their saturated

relatives, the alkanes. The odors of the alkenes are a bit more pungent and perhaps jus-

tify being called “evil-smelling.” In fact, the old trivial name for alkenes, olefins, readily

evokes the sense of smell.Tables 3.2 and 3.3 list some data for alkenes and cycloalkenes.

3.9 Alkynes: Structure and Bonding

Like the simplest alkene,ethylene,the simplest alkyne is generally known by its triv-

ial name, acetylene, not the systematic name, ethyne. Recall that more complicated,

substituted alkynes are often named as derivatives of acetylene. Acetylene itself is a

symmetrical compound of the formula HCCH in which the two carbon atoms are

attached to each other by a triple bond. Each carbon is attached to two other atoms:

one hydrogen and the other carbon.Accordingly, a reasonable bonding scheme must

yield only two hybrid orbitals, one to be used in bonding to the hydrogen, and one

to be used in bonding to the carbon. As in the discussion of alkanes and alkenes

(p. 64; p. 99), the atomic orbitals of carbon are combined to yield hybrid orbitals,

which do a better job of bonding than the unchanged atomic orbitals. Because only

two bonds are needed, one for the bond to hydrogen, and another for the bond to

carbon, we need to combine only two of carbon’s atomic orbitals to give us our

hybrids. A combination of the 2s and 2p

orbitals will yield a pair of sp hybrids.

124 CHAPTER 3 Alkenes and Alkynes

We can anticipate that these new sp hybrids will be directed so as to keep the bonds,

and the electrons in them, as far apart as possible, which produces 180° angles

(Fig. 3.48). (Recall our discussion of BeH

, p. 53.)

2s 2p

Used to make two

sp h

brid orbitals

An sp hybrid

orbital

Two sp hybrids

Not used

180⬚

FIGURE 3.48 To make an sp hybrid we combine the wave functions for the carbon 2s and

orbitals. The 2p

and 2p

orbitals are not used in the hybridization scheme.

FIGURE 3.49 An sp-hybridized

carbon atom. Note the leftover,

unhybridized 2p

and 2p

orbitals.

sp/1s Overlapsp/1s Overlap

C C

sp/sp Overlap

FIGURE 3.50 The σ bonding system

of acetylene: two bonds and

one bond.C

These new sp hybrid orbitals generally resemble the sp

and sp

hybrids we made

before (compare Figure 3.48 with Figures 2.6 and 2.23). Because we used only two of

carbon’s four available atomic orbitals, the 2p

and 2p

orbitals are left over. Figure 3.49

shows an sp-hybridized carbon atom.(From now on, we will not show the small lobes.)

Each carbon–hydrogen bond in acetylene is formed by overlap of one sp hybrid

with the hydrogen 1s orbital, and the carbon–carbon bond is formed by the overlap

of two sp hybrids. This process forms the σ bonds of the molecule (Fig. 3.50).

Figure 3.50 shows the bonds formed by sp/sp and sp/1s overlap, but don’t forget

that these overlapping hybrid and atomic orbitals create both bonding and antibond-

ing molecular orbitals.The empty antibonding orbitals are not shown in Figure 3.50,

but are shown in the schematic construction of Figure 3.51.

Energy

␴*

␴

Energy

␴*

␴

– sp (antibonding)

+ sp (bonding)

–1s (antibonding)

+ 1s (bonding)

C σ Bond C

H σ Bond

FIGURE 3.51 Orbital interaction diagrams showing the bonding and antibonding orbitals formed by sp/sp

and sp/1s overlap.

3.9 Alkynes: Structure and Bonding 125

As in the alkenes, the remaining unhybridized 2p orbitals in the alkynes overlap

to form π bonds. This time there are two p orbitals remaining on each carbon of

acetylene, and we can form a pair of π bonds that are directed, as are the 2p orbitals

making up the π bonds, at 90° to each other (Fig. 3.52).

HHH

HCC

WEB 3D

FIGURE 3.52 Overlap of two 2p

and

two 2p

orbitals forms a pair of π

bonds in alkynes.

Both carbons in acetylene have four valence electrons (Remember: All carbons

have four bonding electrons because we ignore the two very low energy 1s electrons)

and each can participate in four two-electron bonds.In acetylene, these are the σ bond

to hydrogen and the carbon–carbon triple bond composed of one carbon–carbon

σ bond and two carbon–carbon π bonds (Fig. 3.53).

HHC

sp1s

1ssp

CH H

FIGURE 3.53 A highly schematic

construction of the π and σ bonds in

acetylene.

1.54 A

⬚

1.33 A

⬚

1.20 A

⬚

WEB 3D

FIGURE 3.54 Bond lengths in

angstroms (Å) of simple two-carbon

hydrocarbons.

PROBLEM 3.22 Draw the orbital interaction diagrams for construction of the π

bonds of acetylene from the 2p orbitals.

One of the structural consequences of triple bonding is an especially short

carbon–carbon bond distance (⬃1.2 Å), considerably shorter than either

carbon–carbon single or double bonds (Fig. 3.54).

Summary

The σ bonding schemes we used to construct alkanes and cycloalkanes are not

the only way that atoms can be held together to form molecules. In alkenes and

alkynes, there is π bonding as well as σ bonding. In the construction of a π bond,

2p orbitals overlap side-to-side to form a bonding and antibonding pair of π

molecular orbitals. Alkenes and cycloalkenes contain one π bond,whereas alkynes

have two π bonds. Ring compounds can also contain double (and sometimes

triple) bonds.

126 CHAPTER 3 Alkenes and Alkynes

We can start constructing the family of alkynes by replacing X in Figure 3.55

with a methyl group, CH

.This process produces propyne (or methylacetylene), the

lone three-carbon alkyne (Fig. 3.56).

3.10 Relative Stability of Alkynes:

Heats of Formation

Heats of formation show that alkynes are very much less stable than their con-

stituent elements.Table 3.4 lists heats of formation for some alkynes, alkenes, and

alkanes. The virtues of using hybrid orbitals for σ bonding rather than p orbitals

for π bonding are apparent. The more π bonds in a molecule, the more positive

the heat of formation, and the more endothermic the formation of the compound

from its constituent elements.The very positive heats of formation for alkynes have

their practical consequences. When a very high heat is desired, as for many weld-

ing applications, acetylene is the fuel of choice. Acetylenes are high-energy com-

pounds (with correspondingly high heats of formation) and lots of energy is given

off when they react with oxygen to form the much more stable molecules water

and carbon dioxide.

Because of their triple bonds, alkynes can be only mono- or disubstituted.

Disubstituted alkynes are more stable than their monosubstituted isomers. For

example, we can see from Table 3.4 that the heat of formation of 1-butyne is about

5 kcal/mol more positive than that for 2-butyne. This phenomenon echoes the

increasing stability of alkenes with increasing substitution (Section 3.6).

TABLE 3.4 Heats of Formation for Some Small Hydrocarbons

Alkanes Alkenes Alkynes

Ethane 20.1 Ethylene 12.5 Acetylene 54.5

Propane 25.0 Propylene 4.8 Propyne 44.6

Butane 30.2 1-Butene 0.1 1-Butyne 39.5

2-Butene 1.9 (cis) 2-Butyne 34.7

2.9 (trans)

(kcal/mol)≤H

3.11 Derivatives and Isomers of Alkynes

We can imagine replacing one of the hydrogens of acetylene with an atom or group X

to give substituted alkynes. Figure 3.55 gives an example of an “ethynyl”compound. In

practice, two naming systems are used here, and the figure shows them. Chloroethyne

can also be called chloroacetylene, and both naming systems are commonly used.

Chloroethyne

(or chloroacetylene)

X = Cl

FIGURE 3.55 An ethynyl compound,

chloroethyne, a monosubstituted

alkyne.

HCCX

X = CH

HCC

Propyne

(

meth

lacet

lene

)

WEB 3D

FIGURE 3.56 Replacement of X with

a methyl (CH

) group gives propyne.

3.11 Derivatives and Isomers of Alkynes 127

HCC

replace H

XCCCH

HCCCH

1-Propynyl compounds

3-Propynyl compounds

(propargyl compounds)

Propyne

—a

terminal alkyne

replace H

FIGURE 3.57 Examples of the two

kinds of hydrogen in propyne, each

replaced with X to give derivatives.

There are two different hydrogens in propyne, and therefore two different ways to

produce a substituted compound (Fig.3.57).When the acetylenic hydrogen is replaced,

the compounds can be named either as acetylenes or as 1-propynyl compounds.

3-Propynyl compounds result from replacement of a methyl hydrogen with X.

The common name propargyl is reserved for the group and is often

seen. Note the relationship between the allyl (Fig. 3.19) and propargyl groups.

When X  CH

we produce the two four-carbon alkynes, which must be

butynes (Fig. 3.58).

X = CH

HCCCH

X HCCCH

1-Butyne

XCCCH

CCCCH

2-Butyne

FIGURE 3.58 If X  CH

, we get

the two butynes.

The naming protocol for alkynes is similar to that used for alkenes. As with the

double bond of alkenes,the triple bond position is designated by assigning it a num-

ber, which is kept as low as possible. When both double and triple bonds are pres-

ent,the compounds are named as enynes, and the numbers designating the positions

of the multiple bonds kept as low as possible. If the numbering scheme could

produce two names in which the lower number could go to either the “-ene” or the

“-yne,” the “-ene” gets it (Fig. 3.59).

HC C CH

Ethynylcyclohexane

or cyclohexylacetylene

3-Cyclopentylpropyne

or propargylcyclopentane

CCH

HC C CH

1-Butyne

not 3-butyne

3,3-Dimethyl-1-butyne

not 2,2-dimethyl-3-butyne

(E)-2-Hexen-4-yne

trans-2-hexen-4-yne

not (E)-4-hexen-2-yne

23 41

2341

FIGURE 3.59 Some examples of the

naming convention for alkynes.

128 CHAPTER 3 Alkenes and Alkynes

PROBLEM 3.23 Draw all the pentynes, hexynes, and heptynes.

PROBLEM 3.24 Name all the isomers you drew in Problem 3.23.

PROBLEM 3.25 How many signals will appear in the

C NMR spectra of the

compounds in Figure 3.59 (see Section 2.14)?

Can alkynes exhibit cis/trans isomerism? It takes only a quick look to see that

there can be no such isomerism in a linear compound (Fig. 3.60). The sp

hybridization requires 180° angles, and therefore there can be no cis/trans iso-

merism in alkynes. The alkynes are simpler to analyze structurally than the

alkenes.

CCH

180⬚ 180⬚

FIGURE 3.60 There are 180° angles in

2-butyne.

3.12 Triple Bonds in Rings

Like double bonds, triple bonds can occur in rings,although very small ring alkynes

are not known.The difficulty comes from the angle strain induced by the prefer-

ence for linear, 180° bond angles in an acetylene. When a triple bond is incorpo-

rated in a ring, it becomes difficult to accommodate these 180° angles. Deviation

from 180° reduces the overlap between the p orbitals making up one of the π

bonds of the acetylene and that raises the energy of the compound (Fig. 3.61).

Remember: “R” is shorthand for a general group.

bend

A c

clic acet

lene

Second

π bond

in the acetylene

Reduced

overlap

FIGURE 3.61 Incorporation of a triple

bond in a ring is difficult because

orbital overlap between the 2p

orbitals of one of the π bonds is

reduced by the bending required by

the ring.

Cycloheptyne

(observable only at

low temperature)

Cyclooctyne

(stable at

room temperature

but very reactive)

Cyclononyne

(a compound of

normal reactivity)

WEB 3D

FIGURE 3.62 Some cycloalkynes.

A similar problem with angles exists for the cycloalkenes, but it is less severe. In

practice, the smallest ring in which an alkyne is stable under normal conditions is

cyclooctyne. Thus, it and larger cycloalkynes (cyclononyne, etc.) are known, but

the smaller cycloalkynes are either unknown or have been observed only as fleet-

ing intermediates (Fig. 3.62).