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Before we dive into the nitty-gritty details of reactions in organic chemistry, it's important to review a few concepts. Let's get started!
Substrate
Substrate
Certain reactions (such as substitution reactions) center on one carbon atom (perhaps called the alpha carbon or ___ center). The rate of reaction might depend on how many alkyl groups are attached to the carbon. Let's compare the structures of methyl bromide, ethyl bromide, isopropyl bromide, and tert-butyl bromide.
The carbon atom in methyl bromide has no other carbons attached to it, so this molecule is a methyl substrate,
The carbon atom attached to the bromine in ethyl bromide has one other carbon attached to it, so this molecule is a primary (1˚ ) substrate.
The carbon atom attached to the bromine in isopropyl bromide has two other carbons attached to it, so this molecule is a secondary (2˚ ) substrate.
The carbon atom attached to the bromine in tert-butyl bromide has three other carbons attached to it, so this molecule is a tertiary (3˚ ) substrate.
In other words,
no carbons = methyl
one carbon = primary (abbreviated as 1˚)
two carbons = secondary (2˚)
three carbons = tertiary (3˚)
Note that a carbon atom with four other carbons attached is called a quaternary or 4˚ carbon. Such a carbon atom can't also be bonded to another group. Keep in mind also that the rate of a reaction might depend on the substitution (1, 2, or 3˚) of carbons farther away from the site of reaction. For example, even though the alpha carbons in neopentyl halides are primary, neopentyl halides are basically unreactive towards the SN2 mechanism because of the all the steric hindrance that the other carbons provide.
Neopentyl bromide, like other neopentyl halides, is unreactive towards the SN2 mechanism
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