lab 7 - Relative Rates of Bromination of...

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Relative Rates of Bromination of Hydrocarbons/Free-Radical Substitution Mechanism Methods and Background The purpose of the experiment was to determine the relative rates of bromination for different hydrocarbons (toluene, tert -butylbenzene, isopropylbenzene, ethylbenzene, cyclohexane, and methylcyclohexane) and the effect of temperature on the rate of bromination of hydrocarbons. Bond-breaking can occur in two ways: heterolysis and homolysis. In heterolysis, both electrons of a bond are transferred to the more electronegative atom (out of a pair of bonded atoms), resulting in a cation and anion. In homolysis, however, the two atoms have a low electronegativity difference (or no difference at all), so when the bond breaks, one electron is transferred to each atom, resulting in free radicals, atoms with unpaired electrons. In the first step of bromination of alkanes, known as initiation, heat (200-400 °C) or ultraviolet light (hν) applied to a mixture of the alkane and bromine, causing the σ-bond of the molecular bromine (Br 2 ) to undergo homolysis, generating bromine radicals, Br·; Br 2 undergoes homolysis as there is no difference in electronegativity between the identical bromine atoms. Br· are formed in low concentration, resulting in a net increase in the concentration of free radicals within the system. In the next step of bromination of alkanes, propagation, a hydrogen atom is abstracted from the hydrocarbon by a Br· to produce a new free radical, R·, and hydrobromic acid, H-Br. R· then attacks Br 2 to yield the alkyl bromide, R-Br, and Br·, thus regenerating the free radical needed for propagation. In propagation, there is no net change in the concentration of radicals. In the last step of bromination of alkanes, termination, the free radicals formed in the reaction (Br· and R·) combine with each other to give different molecules (Br 2 , R-Br, and R-R), resulting in a net decrease in radical concentration and a decrease in the rate of the reaction.
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Thus, the steps of initiation, propagation, and termination, constitute a free-radical chain mechanism for bromination of alkanes, and the overall reaction can be summarized in the following chemical equation: The drawback of bromination of alkanes is that polysubstitution may occur, however, this can be minimized by using an excess of the alkane undergoing bromination as compared to the amount of Br 2 present. Hydrogen atoms bound to sp 3 -hybridized carbons are categorized as aliphatic, and they are further classified as primary (1°), secondary (2°), or tertiary (3°) if the reference carbon atom is attached to one, two, or three other carbon atoms, respectively. Aliphatic hydrogen atoms whose carbons are bound to vinylic or aromatic carbon atoms are called allylic or benzylic, respectively. Hydrogen atoms attached to
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lab 7 - Relative Rates of Bromination of...

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