Stability of alkenes

Which memher in each of the following sets ranks higher  

Assign E ox Z configuration to the double bonds in the following compounds  

Assign stereochemistry ( or Z] to the double bond in the following compound, and convert the drawing into a skeletal structure (red = O)  

Although the cis-trans interconversion of alkene isomers does not occur spontaneously, it can often be brought about by treating the alkene with a strong acid catalyst. If we interconvert cis-hut-2-ene with trans-but-2-ene and allow them to reach equilibrium, we find that they aren t of equal stability. The trans 

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 mildly exothermic, evolving about 80 to 120 kJ (20 to 30 kcal) of heat per mole of hydrogen consumed. Consider the hydrogenation of 1-butene and rram-2-butene  

An 11 kJ/mol (2.7 kcal/mol) stability difference is typical between a monosubstituted alkene (1-butene) and a trans-disubstituted alkene (rrans-2-butene). In the following 

The most stable double bonds are those with the most alkyl groups attached. For example, hydrogenation of ethylene (no alkyl groups attached) evolves 137 kJ/mol, while propene and 1-pentene (one alkyl group for each) give off 126 kJ/mol. Double bonds with two alkyl groups hydrogenate to produce about 116-120 kJ/mol. Three or four alkyl substituents further stabilize the double bond, as with 2-methyl-2-butene (trisubstituted, 113 kJ/mol) and 2,3-dimethyl-2-butene (tetrasubstituted, 111 kJ/mol). 

The values in Table 7-1 confirm Zaitsev s rule (Saytzeff s rule)  

1-butene CM-2-butene rra s-2-butene 2-methylpropene ----------- increasing stability (decreasing A// )-------------  

The heats of combustion of the isomeric butenes are plotted on the vertical axis in kj mole . AH compounds are at higher energy than the common products, carbon dioxide and water. 

Some of the hydrogen atoms of the two methyl groups in cis-2-butene are within their van der Waals radii. These atoms are in a 1,6 relationship, and they stericaUy interfere with each other. There is no steric effect in the trans isomer. 

Alkenes and cycloalkenes react with hydrogen gas in an addition reaction to give saturated com-p g Q y IQI OF ALKENES po ds. in this process, the alkene is reduced. The reaction is called hydrogenation. The hydrogena- 

However, the hydrogenation of an alkene, such as 1-octene to octane, occurs rapidly at room temperature in the presence of certain transition metal catalysts. One such catalyst is palladium dispersed on carbon (Pd/C). 

Cis alkenes are less stable than their trans isomers because of steric strain between the two larger substituents on the same side of the double bond. This is the same kind of steric interference that we saw previously in the axial conformation of methylcyclohexane (Section 4.7). 

If we were to measure the so-called heats of hydrogenation (AH hydrog) for two double-bond isomers and find their difference, we could determine the relative stabilities of cis and trans isomers without having to measure an equilibrium position. czs-2-Butene, for instance, has Aff hyjjrog = 120 kJ/mol ( 28.6 kcal/mol), while aus-2-butene has A/f°hydrog = -116 kJ/mol (-27.6 kcal/mol)—a difference of 4 kj/mol. 

A second factor that contributes to aikene stability involves bond strengths. A bond between an spi carbon and an s carbon is somewhat stronger than a bond between two sp carbons. Thus, in comparing 1-butene and 2-butene, the mono-substituted isomer has one sp -sf bond and one spi -sp bond, while the disubsti-tuted isomer has two sp -sp bonds. More highly substituted alkenes always have a higher ratio of sp -sp bonds to sp -sp bonds than less highly substituted alkenes and are therefore more stable. 

Name the foiiowing aikenes, and teii which compound in each pair is more stable (a) H2C=CHCH2CH3 or CH3 

For example, ethylene has AF.f hvdr0g = —137 kj/mol (-32.8 kcal/mol), but when one alkyl substituent is attached to the double bond, as in l-butene( the alkene becomes approximately 10 kj/mol more stable (A/-Phydr0g = -126 kj/mol). Further increasing the degree of substitution leads to still further stability. As a general rule, alkenes follow the stability order  

If we were to measure what are called heats of Hydrogenation for 

Using the relationship between equilibrium con.staiit and free energy shown previously in figure 4.12, p. 122, we can calculate that d.s-2-butene is less stable than //uH5-2-butene by 2.8 kJ/mol (0.66 kcal/mol) at room temperature. 

---

Hyperconjugation has also been invoked to account for the greater thermodynamic stability of alkenes in which the double bond is not terminal, e.g. (30), compared with isomeric compounds in which it is, e.g. (31) in (30) there are nine hyperconjugable a-hydrogen atoms, compared with only five in (31) ... 

The differences permit the measurement of the relative stabilities of alkene... 

Blatter, F. and Frei, H. (1993). Very strong stabilization of alkene-02 charge transfer states in zeolite NaY red-light-induced photooxidation of 2,3-dimethyl-2-butene. J. Am. Chem. Soc. 115, 7501-7502... 

In dehydration and dehydrohalogenation the preferential order for removal of an H is 3°>2°> 1 (Saytzeff rule). We can say the poor get poorer. This order obtains because the more R s on the C==C group, the more stable is the alkene. The stability of alkenes in decreasing order of substitution by R is... 

Alkenes are typically nucleophiles, because the double bonds are electron rich and electrons in the it bond are loosely held. Electrophiles are attracted to the TT electrons. Thus, alkenes generally undergo addition reactions, and addition reactions are typically exothermic. The following three factors influence the stability of alkenes. 

In addition to having synthetic applications, catalytic hydrogenation is useful for analytical and thermochemical purposes. The analysis of a substance for the number of carbon-carbon double bonds it contains is carried out by measuring the uptake of hydrogen for a known amount of sample. Measurement of the heat evolved in the hydrogenation of alkenes gives information as to the relative stabilities of alkenes, provided that the differences in AS0 values are small (see Exercise 11-7). 

Hydrogenation is an exothermic reaction with a high energy of activation. Heats of hydrogenation can be used to measure relative stabilities of alkenes. The lower the heat of hydrogenation, the more stable the alkene. 

The Orbital Description of the Alkene Double Bond 286 7-3 Elements of Unsaturation 287 7-4 Nomenclature of Alkenes 289 7-5 Nomenclature of Cis-Trans Isomers 291 Summary Rules for Naming Alkenes 293 7-6 Commercial Importance of Alkenes 294 7-7 Stability of Alkenes 296 7-8 Physical Properties of Alkenes 302... 

This order of preference is the same as the order of stability of alkenes. We consider the stability of alkenes in more detail in Section 7-7, but for now, it is enough just to know that more substituted alkenes are more stable. In Chapter 7, we will study some unusual reactions where Zaitsev s rule does not apply. 

A 4 kJ/mol difference between cis and trans isomers is typical for disubstituted alkenes. Figure 7-8 summarizes the relative stabilities of alkenes, comparing them with... 

Q Predict the relative stabilities of alkenes and cycloalkenes, based on their structure and stereochemistry. 

In Chapter 7, we used heats of hydrogenation to compare the relative stabilities of alkenes. For example, the heats of hydrogenation of pent-l-ene and fran.s-pent-2-ene show that the disubstituted double bond in trans-pent-2-ene is 10 kJ/mol (2.5 kcal/mol) more stable than the monosubstituted double bond in pent-l-ene. 

---