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  Alkanes Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Functional Group Reactions of Alkane Reactions

The cycloalkanes and alkanes, with the exception of cyclopropane, are almost the least chemically reactive class of organic compounds. In spite of their relative inertness, alkane goes through various significant reactions that are considered in the further section.

1. Combustion

The combustion of carbon compounds, mainly hydrocarbons, has been the main significant source of heat energy for human civilizations throughout recorded history. The practical significance of this reaction cannot be denied, but the uncontrolled and enormous chemical changes which take place in combustion make it complex to deduce mechanistic paths. By employing the combustion of propane as an instance, we see from the following equation that every covalent bond in the reactants has been broken and an completely new set of covalent bonds have created in the products. No another common reaction involves this type of a pervasive and profound change, and the method of combustion is so difficult that chemists are just beginning to understand and explore some of its basic characteristics.

CH3-CH2-CH3   +   5 O2   --> 3 CO2   +   4 H2O   +   heat

Two points relating to this reaction are significant:

1. Because in the reactant molecules all the covalent bonds are broken, the quantity of heat evolved in this reaction is associated to the strength of these bonds (and, certainly, the strength of the bonds formed in the products). Specific heats of combustion measurements can provide helpful information about the structure of molecules.
2. The stoichiometry of the reactants is significant. If not sufficient oxygen is supplied some of the products will contained carbon monoxide, a highly toxic gas.

CH3-CH2-CH3   +   4 O2   -->  CO2   +   2 CO   +   4 H2O   +   heat

Heat of Combustion

From the earlier description, we might suppose isomers to have similar heats of combustion. Though, some simple measurements will disabuse this principle. So, the heat of combustion of pentane is -782 kcal/mole, but that of its 2,2-dimethylpropane (neopentane) isomer is -777 kcal/mole. Variations ike this reflect subtle structural variations, which is including the greater bond energy of 1º-C-H versus 2º-C-H bonds and steric crowding of neighboring groups. In the small-ring cyclic compounds ring strain can be a chief contributor to chemical reactivity and thermodynamic stability. The table records heat of combustion data for some simple cycloalkanes and compares these with the increase per CH2 unit for long chain alkanes.

Cycloalkane 
(CH2)n

CH2 Units 
n

ΔH25º 
kcal/mole

ΔH25º 
per CH2 Unit

Ring Strain 
kcal/mole

Cyclopropane

n = 3

468.7

156.2

27.6

Cyclobutane

n = 4

614.3

153.6

26.4

Cyclopentane

n = 5

741.5

148.3

6.5

Cyclohexane

n = 6

882.1

147.0

0.0

Cycloheptane

n = 7

1035.4

147.9

6.3

Cyclooctane

n = 8

1186.0

148.2

9.6

Cyclononane

n = 9

1335.0

148.3

11.7

Cyclodecane

n = 10

1481

148.1

11.0

CH3(CH2)mCH3

m = large

-

147.0

0.0

The main source of ring strain in smaller rings is eclipsing strain and angle strain. As described somewhere else, cyclobutane and cyclopropane have large contributions of both strains, with angle strain being particularly severe. Variations in chemical reactivity as a result of angle strain are dramatic in the case of cyclopropane, and are also manifest for cyclobutane. Some illustrations are displayed in the following picture. The cyclopropane reactions are additions, several of which are started by electrophilic attack. The pyrolytic conversion of β-pinene to myrcene possibly takes place by an initial rupture of the 1:6 bond, giving an allylic 3º-diradical, followed directly by breaking of the 5:7 bond.

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2. Halogenation

Halogenation is the replacement of one or more hydrogen atoms in an organic compound by a halogen (fluorine, chlorine, bromine or iodine). Unlike the complex transformations of combustion, the halogenation of an alkane come into view to be a simple substitution reaction in which a C-H bond is broken and a new C-X bond is formed. The chlorination of methane, displayed below, provides a simple example of this reaction.

CH4   +   Cl2   +   energy   -->  CH3Cl   +   HCl

because only two covalent bonds are divided (C-H & Cl-Cl) and two covalent bonds are formed (C-Cl & H-Cl), this reaction seems to be an perfect example for mechanistic speculation and investigation. Though, one complication is that all the hydrogen atoms of an alkane may go through the substitution, resulting in a mixture of products, as displayed in the unbalanced equation displayed below. The relative amounts of the several products depend on the proportion of the two reactants used. In case of methane, a large excess of the hydrocarbon favors formation of methyl chloride as the main product; where, an excess of chlorine favors formation of carbon and chloroform tetrachloride.

CH4   +   Cl2   +   energy   -->  CH3Cl   +   CH2Cl2   +   CHCl3   +   CCl4   +   HCl

The following factors must be accommodated from any reasonable mechanism for the halogenation reaction.

1. in the following order the reactivity of the halogens decreases: F2 > Cl2 > Br2 > I2.
2. We shall confine our attention to bromine and chlorine, because fluorine is so explosively reactive it is complex to control, and iodine is usually unreactive.
3. Brominations and Chlorinations are generally exothermic.
4. Energy input in the form of heat is essential to initiate these halogenations.
5. If light is employed to start halogenation thousands of molecules react for each photon of light absorbed.
6. Halogenation reactions can be conducted in either the liquid or gaseous phase.
7. The presence of oxygen (a radical trap) inhibits the reaction in gas phase chlorinations.
8. In liquid phase halogenations radical initiators like peroxides facilitate the reaction.

The most reasonable mechanism for halogenation is a chain reaction involving neutral intermediates like free radicals or atoms. In the reactants the weakest covalent bond is the halogen-halogen bond (Cl-Cl = 58 kcal/mole; Br-Br = 46 kcal/mole) so the starting step is the homolytic cleavage of this bond by heat or light, note: bromine and chlorine both absorb visible light (they are colored). A chain reaction technique for the chlorination of methane has been elaborated.
Bromination of alkanes takes place by an identical mechanism, but is slower and more selective since a bromine atom is a less reactive hydrogen abstraction agent than a chlorine atom, as imitated by the higher bond energy of H-Cl than H-Br.

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