Alkenes Structure and Preparation Elimination

Chapter 5

Alkenes I. Structure and Preparation Elimination

Unsaturated hydrocarbons

In our discussion of the alkanes, we mentioned briefly another family of hydrocarbons, the alkenes, which contain less hydrogen, carbon for carbon, than the alkanes, and which can be converted into alkanes by addition of hydrogen. The alkenes were further described as being obtained from alkanes by loss of hydrogen in the cracking process. Since alkenes evidently contain less than the maximum quantity of hydrogen, they are referred to as unsaturated hydrocarbons. This unsaturation can be satisfied by reagents other than hydrogen and gives rise to the characteristic chemical properties of alkenes.

Structure of ethylene. The carbon-carbon double bond

The simplest member of the alkene family is ethylene, C2 H4. In view of the ready conversion of ethylene into ethane, we can reasonably expect certain structural similarities between the two compounds. To start, then, we connect the carbon atoms by a covalent bond, and then attach two hydrogen atoms to each carbon atom. At this stage we find that each carbon atom possesses only six electrons in its valence shell, instead of the required eight, and then the entire molecule needs an additional pair, of electrons if it is to be neutral. We can solve both these problems by assuming that the carbon atoms can share two pairs of electrons. To describe this sharing of two pairs of electrons, we say that the carbon atoms are joined by a double bond. The carbon-carbon double bond is the distinguishing feature of the alkene structure.

The molecule is not yet complete, however. In forming the sp 2 orbitals, each carbon atom has used only two of its three p orbitals. The remaining p orbital consists of two equal lobes, one lying above and the other lying below the plane of the three sp 2 orbitals it is occupied by a single electron. If the/> orbital of one carbon atom overlaps the p orbital of the other carbon atom, the electrons pair up and an additional bond is formed.

This quantum mechanical structure of ethylene is verified by direct evidence. Electron diffraction and spectroscopic studies show ethylene to be a flat molecule, with bond angles very close to 120. The CC distance is 1.34 A as compared with the CC distance of 1.53 A in ethane.

Propylene

The next member of the alkene family is propylene, C3 H6. In view of its great similarity to ethylene, it seems reasonable to assume that this compound also contains a carbon-carbon double bond. Starting with two carbons joined by a double bond and attaching the other atoms according to our rule of one bond per hydrogen and four bonds per carbon, we arrive at the structure H H H H~C~C=C-H i Propylene.

Hybridization and orbital size

The carbon-hydrogen bonds of ethylene are single bonds just as in, say, ethane, but they are formed by overlap of spa 2 orbitals of carbon, instead of sp 3 orbitals as in ethane. Now, compared with an sp 3 orbital, and spa 2 orbital has less p character and more s character. A p orbital extends some distance from the nucleus; an s orbital, on the other hand, lies close about the nucleus. As the s character of a hybrid orbital increases, the effective size of the orbital decreases and, with it, the length of the bond to a given second atom. Thus, a spa 2-s carbon-hydrogen bond should be shorter than an spa 3-s carbon-hydrogen bond.Benzene, in most ways a quite different kind of molecule from ethylene (Sec. 10.1), also contains sp 2 -s carbon-hydrogen bonds; the C~-H bond distance is 1.084 A, almost exactly the same as in ethylene. Acetylene (Sec. 8.2) contains. s/j-hybridized carbon which, in view of the even greater s character of the orbitals, should form even shorter bonds than in ethylene; this expectation is correct, the sp-s bond being only 1 .079 A.A consideration of hybridization and orbital size would lead one to expect an sp 2-sp* bond to be shorter than an sp*-sp* bond. In agreement, the Carbon single bond-distance in propylene is 1.501 A, as compared with the Carbon distance of 1.534 A in ethane. The so-so* carbon-carbon single bond in methylacetylene (Sec. 8.19) is even shorter, 1.459 A. These differences in carbon single bond lengths are greater than the corresponding differences in carbon-hydrogen bond lengths; however, another factor (Sec. 8.18) besides the particular hybridization of carbon may be at work.

Th4 butene's

Going on to the butylene's, C4 Ha, we find that there are a number of possible arrangements. First of all, we may have a straight-chain skeleton as in //-butane, or a branched-chain structure as in isobutane. Next, even when \\e restrict us to the straight-chain skeleton, we find that there are two possible arrangements that differ in position of the double bond in the chain. So far, then, we have a total of three structures; as indicated, these are given the names 1-butene, 2-butene, and isobutylene.

To understand the kind of isomerism that gives rise to two 2-butenes, we must examine more closely the structure of alkenes and the nature of the carbon-carbon double bond. Ethylene is a flat molecule. We have seen that this flatness is a result of the geometric arrangement of the bonding orbitals, and in particular the overlap that gives rise to the IT orbital. For the same reasons, a portion of any alkene must also be flat, the two doubly bonded carbons and the four atoms attached to them lying in the same plane.

which the atoms can be arranged (aside from the infinite number of possibilities arising from rotation about the single bonds). In one of the structures the methyl groups lie on the same side of the molecule (I), and in the other structure they lie on opposite sides of the molecule (II).

Now the question arises: can we expect to isolate two isomeric 2-butenes corresponding to these tv\o different structures, or are they too readily interconverted like, say, the conformations of //-butane (Sec. 3.5)? Conversion of 1 into II involves rotation about the carbon-carbon double bond. The possibility of isolating isomers depends upon the energy required for this rotation. We have seen that the formation of the rr bond involves overlap of the p orbitals that lie above and below the plane of the a orbitals. To pass from one of these 2-butenes to the other, the molecule must be twisted so that the p orbitals no longer overlap; that is, the n bond must be broke.

Geometric isomerism

Since the isomeric 2-butencs dieters from one another only in the way the atoms are oriented in space (but are like one another with respect to which atoms are attached to which other atoms), they belong to the general class we have called stereoisomers. They are not, however, mirror images of each other, and hence are not enantiomers. As we have already said, stereoisomers that are not mirror images of each other are called diastereomer's. The particular kind of diastereomers that owe their existence to hindered rotation about double bonds are called geometric isomers. The isomeric 2-butenes, then, are diastereomers, and more specifically, geometric isomers.

We recall that the arrangement of atoms that characterizes a particular stereoisomer is called its configuration. The configurations of the isomeric 2-butenes are the structures I and II. These configurations are differentiated in their names by the prefixes cis- (Latin: on this side) and trans- (Latin: across), which indicate that the methyl groups are on the same side or on opposite sides of the molecule. In a way that we are not prepared to take up at this time, the isomer of b.p. +4 has been assigned the cis configuration and the isomer of B.Pd. + 1 the trans configuration.

As the examples above illustrate, geometric isomers have different physical properties: different melting points, boiling points, refractive indices, solubilities, densities, and so on. On the basis of these different physical properties, they can be distinguished from each other and, once the configuration of each has been determined, identified. On the basis of these differences in physical properties they can, in principle at least, be separated. When we take up the physical properties of the alkenes (Sec. 5.9), we shall discuss one of the ways in which we can tell whether a particular substance is the eels- or /rows-isomer, that is, one of the ways in which we assign configuration.

Higher alkene

As we can see, the butylene's contain one carbon and two hydrogens more than propylene, which in turn contains one carbon and two hydrogens more than ethylene. The alkenes, therefore, form another homologous series, the increment being the same as for the alkanes: CH2. The general formula for this family is Cn H2n. As we ascend the series of alkenes, the number of isomeric structures for each member increases even more rapidly than in the case of the alkane series; in addition to variations in the carbon skeletons, there are variations in the position of the double bond for a given skeleton, and the possibility of geometric isomerism. Problem 5.1 Neglecting enantiomers', draw structures of: (a) the six isomeric phenylenes (C5 H|); (b) the four chloropropyl Enes (C3 H5 C1); (c) the eleven chlorobactenes' (C4H7C1). Specify as Z or E each geometric.

Names of alkene

Common names are seldom used except for three simple alkenes: ethylene,

propylene, and isobutylene. The various alkenes of a given carbon number are,

however, sometimes referred to collectively as the pentalene's (amylenes), he xylenes,

heptyl Enes, and so on. (One sometimes encounters the naming of alkenes as derivatives of ethylene: as, for example, tetramethyl ethylene for (CH3) 2C C(CH3)2 .)

Most alkenes are named by the IUPAC system.

The rules of the IUPAC system are:

1 . Select as the parent structure the longest continuous chain that contains the

carbon-carbon double bond; then consider the compound to have been derived

from this structure by replacement of hydrogen by various alkyl groups. The

parent structure is known as ethene<propene,butene,pentene, and so on, depending

upon the number of carbons atoms, each name is derived by changing the ending

-ani of the corresponding alkane name to -Ene:

2. Indicate by a number the position of the double bond in the parent chain. Although the double bond involves two carbon atoms, designate its position by the number of the first doubly-bonded carbon encountered when numbering from the end of the chain nearest the double bond; thus J-butene and 2-butene.

3. Indicate by numbers the positions of the alkyl groups attached to the parent chain

Physical properties

As a class, the alkenes possess physical properties that are essentially the same as those of the alkanes. They are insoluble in water, but quite soluble in nonpolar solvents like benzene, ether, chloroform, or ligroin. They are less dense than water. As we can see from Table 5.2, the boiling point rises with increasing.

number; as with the alkanes, the boiling point rise is 20 30" for each added carbon, except for the very small homologs. As before, branching lowers the boiling point. A comparison of Table 5.2 with Table 3.3 (p. 86) shows that the boiling point of an alkene is very nearly the same as that of the alkane with the corresponding carbon skeleton. Like alkanes, alkenes are at most only weakly polar. Since the loosely held rr electrons of the double bond are easily pulled or pushed, dipole moments are larger than for alkanes. They are still small, however: compare the dipole moments shown for propylene and 1-butene, for example, with the moment of 1.83 r> for methyl chloride. The bond joining the alkyl group to the doubly bonded carbon has a small polarity, which is believed to be in the direction shown, that is, with the alkyl group releasing electrons to the doubly bonded carbon. Since this polarity is not canceled by a corresponding polarity in the opposite direction, it gives a net dipole moment to the molecule.

The relationship between configuration and boiling point or melting point is only a rule of thumb, to which there are many exceptions (for example, the boiling points of the diiodoethanes). Measurement of dipole moment, on the other hand, frequently enables us positively to designate a particular isomer as c/$ or trans.

Industrial

Alkenes are obtained in industrial quantities chiefly by the cracking of petroleum (). The smaller alkenes can be obtained in pure form by fractional distillation and are thus available for conversion into a large number of important aliphatic compounds. Higher alkenes, which cannot be separated from the complicated cracking mixture, remain as valuable components of gasoline. 1-Alkenes of even carbon number, consumed in large quantities in the manufacture of detergents, are available through controlled ionic polymerization of ethylene by the Ziegler-Natta method.

Preparation

Alkenes containing up to five carbon atoms can be obtained in pure form from the petroleum industry. Pure samples of more complicated alkenes must be prepared by methods like those outlined below. The introduction of a carbon-carbon double bond into a molecule containing only single bonds must necessarily involve the elimination of atoms or groups from two adjacent carbons: -C-C- * C=-C~ Elimination I I Y Z In the cracking process already discussed, for example, the atoms eliminated are both hydrogen atoms: C- C J^ C--C + H2 I : H H The elimination reactions described below not only can be used to make simple alkenes, but also and this is much more important provide the best general ways to introduce carbon-carbon double bonds into molecules of all kinds.

Dehalogenation of vicinal (Latin: vicinal is, neighboring) dihalides is severely limited by the fact that these dihalides are themselves generally prepared from the alkenes. However, it is sometimes useful to convert an alkene to a dihalide while we perform some operation on another part of the molecule, and then to regenerate the alkene by treatment with zinc; this procedure is referred to as protecting the double.

Dehydrohalogenation of alkyl halides

Alkyl halides are converted into alkenes by dehydrohalogenation: elimination of the elements of hydrogen halide. Dehydrohalogenation involves removal of the halogen atom together with a hydrogen atom from a carbon adjacent to the one Dehydrohalogenation: elimination of HX i I II C C + KOH (alcoholic) - > C=C - + KX + H2O ' I Alkene H A. Alky] halide bearing the halogen. It is not surprising that the reagent required for the elimination of what amounts to a molecule of acid is a strong base. The alkene is prepared by simply heating together the alkyl halide and a solution of potassium hydroxide in alcohol. For example:

As we can see, in some cases this reaction yields a single alkene. and in other cases, yields a mixture. w-Butyl chloride, for example, can eliminate hydrogen only from C-2 and hence yields only 1-butene. sec-Butyl chloride, on the other hand, can eliminate hydrogen from either C-l or C-3 and hence yields both 1-butene and 2-butene. Where the two alkenes can be formed, 2-butene is the chief product; this fact fits into a general pattern for dehydrohalogenation which is discussed in.

Mechanism of dehydrohalogenation

The function of hydroxide ion is to pull a hydrogen ion away from carbon; simultaneously a halide ion separates and the double bond forms. We should be

notice that, in contrast to free radical reactions, the breaking of the CH and C X bonds occurs in an unsymmetrical fashion: hydrogen relinquishes both electrons to carbon, and halogen retains both electrons. The electrons left behind by hydrogen are now available for formation of the second bond (the TT bond) between the carbon atoms. What supplies the energy for the breaking of the carbon-hydrogen and carbon-halogen bonds? (a) First, there is formation of the bond between the hydrogen ion and the very strong base, hydroxide ion. (b) Next, there is formation of the TT bond which, although weak, does supply about 70 kcal/mole of energy.

Orientation and reactivity in dehydrohalogenation

In cases where a mixture of isomeric alkenes can be formed, which isomer, if any, will predominate? Study of many dehydrohalogenation reactions has shown that one isomer generally does predominate, and that it is possible to predict which isomer this v\ill be- that is, to predict the orientation of elimination -on the basis of molecular structure.

In dehydrohalogenation, the more stable the alkene the more easily it is formed. Examination of the transition state involved shows that it is reasonable that the more stable alkene should be formed fasted

Carbonium ions

To account for the observed facts, we saw earlier, a certain mechanism was advanced for the halogenation of alkanes; the heart of this mechanism is the fleeting existence of free radicals, highly reactive neutral particles bearing an odd electron. Before we can discuss the preparation of alkenes by dehydration of alcohols, we must first learn something about another kind of reactive particle: the carbonium ion, a group of atoms that contains a carbon atom bearing only six electrons. Carbonium ions are classified as primary, secondary, or tertiary after the carbon bearing the positive charge. They are named by use of the word cation. For example: H H H CH3 H:C CH,:Co CH3: C:CH3 CH3: C:CH3 H H ' @ Methyl cation Ethyl cation Isopropyl cation /e/7-Butyl cation (primary, 1") (secondary, 2) (tertiary, 3).

In 1963, George Olah (now at Case Western Reserve University) reported the direct observation of simple alkyl cations. Dissolved in the extremely powerful Lewis's acid SbF5, alkyl fluorides (and, later, other halides) were found to undergo ionization to form the cation, which could be studied at leisure. There was a RF + SbF5 > R+ SbF6 - dramatic change in the nm. spectrum (Chap. 13), from the spectrum of the alkyl fluoride to the spectrum of a molecule that contained no fluorine but instead j/? 2-hybridized carbon with a very low electron density. Figure 5.7 shows what was observed for the tern-butyl fluoride system: a simple spectrum but, by simplicity, enormously significant. Although potentially very reactive, the two/- butyl cation can do little in this environment except try to regain the fluoride ion and the SbF5 is an even stronger Lewis's acid than the.

Structure of carbonium ions

In a carbonium ion, the electron-deficient carbon is bonded to three other atoms, and for this bonding uses v/? - orbitals; the bonds are trigonal, directed to the corners of an equilateral triangle. This part of a carbonium ion is therefore flat, the electron-deficient carbon and the three atoms attached to it lying in the same.

'Fher can be little doubt that carbonium ions actually are flat. The quantum mechanical picture of a carbonium ion is exactly the same as that of boron trifluoride (Sec. 1JO). a molecule whose flatness is firmly established. Namr and infract spectra of the stabilized carbonium ions studied by Olah are consistent with A/> : Inbody/anion and flatness: in particular, infrared and Raman spectra of the /IT/- bye cation are strikingly similar to those of trimethyl boron, known to be flat.

F- \idence of another kind indicates that carbonium ions not only normally

arc flat, but ha\c a stiong necJ to 'he flat. Consider the three tertiary alkyl bromides:

/</7-but\l bromide: and I and II, which are bicyclic (Tavo-ringed) compounds bromine at the bridgehead. The impact of a high-energy electron can remove bromine from an alkyl bromide and generate a carbonium ion; the energy of the electron required to do the job can be measured. On electron impact, I require 5 kcal mole more energy to form the carbonium ion than does /e/7-butyl bromide, and II requires 20 kcal mole more energy. Mo\\ arc we to interpret these facts? On conversion into a carbonium ion, three carbons must move into the plane of the electron-deficient carbon: easy for

the open-chum /cry-butyl group; but difficult for F, where the three carbons are tied back by the ring system; and still more difficult for II, where they are tied back more tightly by the smaller ring. Imagine* or, better, make a model of I or II. You could squash the top of the molecule flat, but only by distorting the angles of the other bonds away from their normal tetrahedral angle, and thus introducing angle strain (Sec. 9.7). Now, why is there this need to be flat? Partly, to permit formation of the strongest possible a bond through sp 2 hybridization.

Stability of carbonium ions. Accommodation of charge

An electron-releasing substituent tends to reduce the positive charge at the electron-deficient carbon; in doing this, the substituent itself becomes somewhat positive. This dispersal of the charge stabilizes the carbonium ion. An electron-withdrawing substituent tends to intensify the positive charge on the electron-deficient carbon, and hence makes the carbonium ion less stable. We consider electronic effects to be of two kinds: inductive effects, related to the electronegativity of substituents; and resonance effects. In the case of carbonium ions, we shall see, a resonance effect involves overlap of the "empty" p orbital of the electron-deficient carbon with orbitals on other, nearby atoms; the result is, of course, that the p orbital is no longer empty, and the electron-deficient carbon no longer so positive. Maximum overlap depends on coplanarity in this part of the molecule, and it is here that we find the second advantage of flatness in a carbonium ion. So far, we, have discussed only factors operating within a carbonium ion to make it more or less stable than another carbonium ion. But what is outside the carbonium ion proper its environment can be even more important in determining how fast a carbonium ion is formed, how long it lasts, and what happens to it. There are onions, one of which may stay close by to form an ion pair. There is the solvent: a cluster of solvent molecules, each with the positive end of its dipole turned toward the cation; possibly one solvent molecule or two playing a special role through overlap of one or both lobes of the p orbital. There may be a neighboring group effect (Chap. 28), in which a substituent on a neighboring carbon approach closely enough to share its electrons and form a covalent bond: an internal factor, actually, but in its operation much like an external factor. In all this we see the characteristic of carbonium ions that underlies their whole pattern of behavior: a needfire electrons to complete the octet of carbon.

Relative stabilities of alkyl cations

The amount of energy required to remove an electron from a molecule or atom is called the ionization potential. (It is really the ionization energy.) The ionization potential of a free radical is, by definition, the A// for the conversion of the radical into a carbonium ion: R. ^ R+ + e - A H ~ ionization potential In ways that we cannot go into, the ionization potentials of many free radicals have been measured. For example:

Now, by definition, the distinction among primary, secondary, and tertiary cations is the number of alkyl groups attached to the electron-deficient carbon. The facts are, then, that the greater the number of alkyl groups, the more stable the carbonium ion.

If our generalization about dispersal of charge applies in this case, alkyl groups must release electrons here: possibly through an inductive effect, possibly through resonance.

Dehydration of alcohols

Alcohols are compounds of the general formula, ROH, where R is any alkyl group: the hydroxyl group, OH, is characteristic of alcohols, just as the carbon carbon double bond is characteristic of alkenes. An alcohol is named simply by naming the alkyl group that holds the hydroxyl group and following this by the word alcohol. It is classified as primary (\). secondary (2), or tertiary (3), depending upon the nature of the carbon atom holding the hydroxyl group . For example:

Where isomeric alkenes can be formed, we again find the tendency for one isomer to predominate. Thus, sec-butyl alcohol, which might yield both 2-butene and 1-butene, actually yields almost exclusively the 2-isomer (). The formation of 2-butene from //-butyl alcohol illustrates a characteristic of dehydration that is not shared by dehydrohalogenations: the double bond can be formed at a position remote from the carbon originally holding the OH group. This characteristic is accounted for later. It is chiefly because of the greater certainty as to where the double bond will appear the dehydrohalogenation is often preferred over dehydration as> a method of making.

Mechanism of dehydration of alcohols

The generally accepted mechanism for the dehydration of alcohols is summarized in the following equations: for the sake of simplicity, ethyl alcohol is used as the example. The alcohol unites (step 1) with a hydrogen ion to form the protonated alcohol, which dissociates (step 2) into water and a carbonium ion; the carbonium ion then loses (step 3) a hydrogen ion to form the

base, H2O, to form the oxonium ion, H3O+; the basic properties of each are due of course, to the unshared electrons that are available for sharing with the hydrogen ion. An alcohol also contains an oxygen atom with unshared electrons and hence displays basicity comparable to that of water. The first step of the mechanism is more properly represented a

Ease of formation of carbonium ions

The ease with which alcohols undergo dehydration follows the sequence 3 > 2 > 1. There is evidence that a controlling factor in dehydration is ihe formation of the carbonium ion, and that one alcohol is dehydrated more easily than another chiefly because it forms a carbonium ion more easily. Carbonium ions can be formed from compounds other than alcohols, and in reactions other than elimination. In all these cases the evidence indicates that the ease of formation of carbonium ions follows the same sequence: Ease of formation of carbonium ions 3 > 2 > 1 > CH3 + In listing carbonium ions in order of their ease of formation, we find that we have at the same time listed them in order of their stability. The more stable the carbonium ion, the more easily it is formed. Is it reasonable that the more stable carbonium ion should be formed more easily? To answer this question, we must look at a reaction in which a carbonium ion is formed and consider the nature of the transition state. In the dehydration of an alcohol, the carbonium ion is formed by loss of water from the protonated alcohol, ROH2 *, that is, by breaking of the carbon oxygen bond. In the reactant the positive charge is mostly on oxygen, and in the product, it is on carbon. In the transition state the C O bond must be partly broken, oxygen having partly pulled the electron pair away from carbon. The positive charge originally on oxygen is now divided between carbon and oxygen. Carbon has partly gained the positive charge it is to carry in the final carbonium ion

Rearrangement of carbonium ions

Very often, dehydration gives alkenes that do not fit the mechanism as we have so far seen it. The double bond appears in unexpected places; sometimes the carbon skeleton is even changed. For example:

In the case of the w-butyl cation, a shift of hydrogen yields the more stable m'-butyl cation; migration of an ethyl group would simply form a different w-butyl cation. In the case of the 2-methyl-l-butyl cation, a hydride shift yields a tertiary cation, and hence is preferred over a methyl shift, which would only yield a secondary cation. In the case of the 3,3-dimethyl-2-butyl cation, on the other hand, a methyl shift can yield a tertiary cation and is the rearrangement that takes.

Orientation and reactivity in dehydration

At this point, we know this much about dehydration of alcohols. (a) It involves the formation of a carbonium ion. How fast dehydration takes place depends chiefly upon how fast this carbonium ion is formed, which, in turn, depends upon how Stable the carbonium ion is. The stability of the carbonium ion depends upon the dispersal of the positive charge, which is determined by electron-release or electron-withdrawal by the attached groups. (b) If this initially formed carbonium ion can rearrange via a 1,2-shift to form a more stable carbonium ion, it will do so. This brings us to the last step of dehydration, (c) The carbonium ion either the original one or the one formed by rearrangement loses a proton to form an alkene. Now, if isomeric alkenes can be formed in this step, which, if any, will predominate? The examples we have already encountered give us the answer: CH3CH2CHCH3 > CH3CH=HCH3 and CH3CH2CH=CH2 2-Butene Preferred product 1-Butene CH3 CH3 CH3 CH3CH2CCH3 CH3CH=CCH3 and CH3CH2O=CH2 2-Methyl-2-butene 2-Methyl-l -butene Preferred product CH3 CH3 CH3 CH3 CH3 CH3 CH3C CCH3 > CH3C==CCH3 and CH3C ^CH2 I I Nu IJT n 2,3-Dimethyl-2-butene 2,3-Dimethyl-l.

As the proton is pulled away by the base (the solvent), the electrons it leaves behind become shared by the two carbons, and the carbon-carbon bond acquires double bond character. Factors that stabilize an alkene also stabilize an incipient alkene in the transition state.