Alkyl Halides
Properties & Synthesis
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| Unit 8 Alkyl Halides Properties & Synthesis |
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Overview
We begin our study of alkyl halides by briefly reviewing what constitutes the initiation of a reaction sequence: the breaking of a covalent hydrocarbon bond. This can occur i none of two fundamentally different ways, depending on what happens to the two electrons making up the bonding pair.
1) Homolysis is chemical bond dissociation of a neutral molecule generating two species called free radicals, which are typically associated with single unpaired electrons. That is, the two electrons that are involved in the bond are distributed one by one to the two species.
A : B − > A. + B.
The energy involved in this process is called bond dissociation energy. Recall that we saw distinct examples of this type in Unit 1-- the discussion of the halogenation reaction sequence of methane and other alkanes.
2) Heterolysis is chemical bond cleavage of a neutral molecule generating a positively charged cation and a negatively charged anion. In this process the two electrons that make up the bond are assigned to the same fragment.
A : B --> A + + B: -
Thus, heterolytic reactions are those in which the bonding electrons are taken away -- or provided -- in pairs. The energy involved in this process is called heterolytic bond dissociation energy. In heterolysis, additional energy is required to separate the ion pair. An ionizing solvent helps reduce this energy. In summary:
1) Homolytic chemistry is the chemistry of the odd electron.
2) Heterolytic chemistry is the chemistry of the electron pair.
Homolytic reactions are typically carried out in the gas phase, or in solvents whose principal function is to provide an inert medium in which the reactants can move about freely (mobility).
Heterolytic reactions are typ0ically carried out in solution, and the solvents exert powerful effects.
So far, the reactions we have been chiefly concerned with were characterized by homolytic bond cleavage. We now begin our study of heterolytic chemistry -- which constitutes the vast majority of chemical reactions involving hydrocarbon compounds. The reaction we shall start with is a substitution reaction. But unlike alkane halogenation, this reaction is heterolytic. It is based on substitution of one nucleophile by another, and applies only to aliphatic (vs. aromatic) compounds.
We call it nucleophilic aliphatic substitution.
Classification & Nomenclature
We classify a carbon atom as primary, secondary, or tertiary according to the number of other carbon atoms attached to it. An alkyl halide is classified according to the kind of carbon that bears the halogen atom.
As members of the same family, containing the same functional group, alkyl halides of different classes tend to undergo the same kinds of reactions. They differ in rates of reaction, however, and these differences in rates may lead to other differences as well.
Alkyl halides can be given two kinds of names: common names (for the simpler halides) and IUPAC names, in which the compound is named as an alkane with a halogen attached as a side chain. For example:
~~~~~~ Physical Properties ~~~~~~~~~~
Because of their greater molecular weights, alkyl halides have considerably higher boiling points than unhalogenated alkanes with the same number of carbons. For a given alkyl group, the boiling point increases with increasing atomic weight of the given halogen, so that a fluoride is the lowest boiling and an iodide the highest. For a given halogen, the boiling point rises with increasing carbon number. Branching of any kind (R group or halogen X) lowers the boiling point.
In spite of their modest polarity, alkyl halides are insoluble in water, probably because of their inability to form hydrogen bonds. They are soluble in the typical organic solvents of low polarity, like benzene, ether, chloroform, or ligroin (petroleum ether). Iodo, bromo, and polychloro compounds are more dense than water ( > 1.0 g / cc). Thus, as molecules of low polarity, both the alkanes and alkyl halides are held together by van der Waals forces or weak dipole-dipole attractions. The are characterized by low melting points and boiling points, and are soluble in non-polar solvents.
~~~ Preparation / Synthesis ~~~
I. Halogenation of Alkanes
As we might expect, halogenation of the higher alkanes is essentially the same as the halogenation of methane. It can be complicated, however, by the formation of mixtures of isomers.
Under the influence of UV light or temperatures from 250 to 400 degrees C, chlorine or bromine converts alkanes into chloroalkanes (alkyl chlorides) or bromoalkanes (alkyl bromides). Reaction rates of the chlorination and bromination reactions are similar. When diluted with an inert gas, and with sufficient heat transfer for cooling, (in contrast to methane) fluorine has recently been found to give analogous results. But as with methane, iodination does not take place at all.
[*Note: An alkyl iodide is often prepared from the corresponding bromide or chloride by treatment with a solution of sodium iodide in acetone. The less soluble sodium bromide or sodium chloride precipitates from solution and can be removed by filtration.]
Isomeric Products
Depending on which hydrogen atom is replaced, any of number of isomeric products can be formed from a single alkane. Thus, free-radical (homolytic) chlorination leads to a possible substitution at each C atom that bears an H atom. This situation essentially requires the recognition of structures that contain various numbers of non-equivalent H atoms. Since all H atoms are potential candidates for chorination, then in general, the number of non-equivalent H atoms is equal to the number of different possible substitution sites -- and thus the number of potential isomers. For example:
1) Ethane can yield only one alkyl halide (all H atoms are equivalent)
2) Propane, n-butane and isobutane can yield two isomers each.
3) n-Pentane can yield three isomers.
4) Isopentane can yield four isomers.
Experiment has shown that on halogenation, an alkane yields a mixture of all possible isomeric products. These results indicate that all H atoms are eligible for replacement.
For example, for chlorination:
Bromination gives the corresponding bromides, but in greatly different proportions:
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*** Example Problem: Among the isomeric alkanes of molecular formula C5H12, identify the one that on photochemical chlorination yields the alkyl chloride specified in each case.
a) A single monochloride
2,2 - Dimethylpropane (neopentane)
Notice that in this highly symmetrical (totally non-polar) structure, all H atoms are equivalent. Thus only one possible monochloride is possible. The same isomer will result from the substitution of any of these H atoms.
1 - Chloro - 2,2 - dimethylpropane
b) 3 isomeric monochlorides
The simplest constitutional isomer neopentane, the straight-chain n-Pentane, satisfies the C5H12 chemical formula requirement with 3 non-equivalent hydrogen sites.
n - Pentane
Chlorination of n-Pentane at the following 3 non-equivalent hydrogen sites yields three different structural (constitutional) isomers of monochloropentane.
1 - Chloropentane
2 - Chloropentane
3 - Chloropentane
c) 4 isomeric monocholrides
Another structural isomer of neopentane, 2 - Methylbutane, satisfies the C5H12 chemical formula requirement with 4 non-equivalent hydrogen sites.
First of all, note the equivalence of the following 2 substitution sites. [Note that the C atom in either of these methyl groups could be considered as the first in the chain (C1)].
1 - Chloro - 2 - methylbutane
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The other 3 non-equivalent sites for substitution in 2 -Methylbutane are in the methyl groups associated with carbon atoms C2, C3 and C4.
1- Chloro - 2 - methylbutane
2 - Chloro - 3 - methylbutane
1 - Chloro - 3 - methylbutane
d) 2 isomeric dichlorides
In order to identify the alkanes of molecular formula C5H12 which photochemical chlorination yield 2 isomeric dichlorides, the starting alkane must have a structure that is rather symmetrical; that is, one in which most or all of the hydrogen substitution (or chlorination) sites are equivalent. The molecule from part a) that satisfies this requirement is:
2,2 Dimethylpropane
Again, we first note the similarities of the following 2 identical structures in 3-dimensions.
1,3 - Dichloro - 2,2 - dimethylpropane
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We then illustrate the only other non-equivalent H atom substitution site as follows:
1,3 - Dichloro - 2,2 - dimethylpropane
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Based on these results, we must therefore conclude that the method of halogenation of alkanes is not a suitable technique for the laboratory preparation of alkyl halides.
We turn now to more appropriate methods of preparation and synthesis.
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II. Synthesis from Alcohols
Alkyl halides are nearly always prepared form alcohols. The reaction mechanism is a nucleophilic substitution SN1 as described in Unit 6: Alcohols.
In the laboratory, alcohols are the most common starting point for the synthesis of other aliphatic compounds. One of the most common first steps in such a synthesis is the conversion of the alcohol into an alkyl halide. Once the alkyl halide is made, the synthesis can follow any one of the myriad of possibilities available via nucleophilic aliphatic substitution -- proceeding via either the SN1 or SN2 mechanism as described herein.
** Note: Alcohols can also be synthesized by the:
1) Addition of hydrogen halides to alkenes (as described in Unit 9: Alkenes)
2) Addition of halogens to alkenes (as described in Unit 9: Alkenes)
3) Addition of halogens to alkynes (as described in Unit 10: Alkynes)
