Question #539df

2 Answers
Jun 2, 2015

The size of the group decides whether a molecule is bulky or not.

Consider the methane molecule. It has 4 C-H bonds arranged in the 3D space and having a bond angle of 109.5 degrees.Note that these bonds are free to rotate in space as long as they remain in tetrahedral geometry.

Now i replace the three Hydrogen atoms with Methyl groups.Try to imagine the structure of the molecule- the same carbon at the centre attached to 4 carbons which in turn are attached to the 3 hydrogen atoms each.

Look at methane and see how the H are arranged in the entire 360-degree space.Now for the new molecule try to imagine how much space is available for the 12 hydrogen atoms (3 from 4 carbons) to rotate.Hardly any.In fact they will try to repel each other off.This is how we define a bulky group.Bulky groups do not allow the entry of another atom from another molecule easily(since they leave you no space to approach the central atom).

Jun 2, 2015

Quick answer: The t-butyl group is a bulky group.

"Bulky" is a relative term. The real question is, "Bulky compared to what?"

The bulk of a substituent is often measured by its effect on the rate of an #"S"_"N"2# substitution reaction.

This gives an order of bulkiness:

#underbrace("CH"_3-)_color(red)("methyl") < underbrace("CH"_3"CH"_2-)_color(red)("ethyl") < underbrace(("CH"_3)_2"CH"-)_color(red)("isopropyl") < underbrace(("CH"_3)_3"CCH"_2-)_color(red)("neopentyl") < underbrace(("CH"_3)_3"C"-)_color(red)(t-"butyl")#

#"E2"# reactions are favoured by strong bulky bases.

The typical bulky base is t-butoxide ion, #("CH"_3)_3"CO"^-#.

Other bulky bases are triethylamine, #("CH"_3"CH"_2)_3"N"#, diisopropylamine, #(("CH"_3)_2"CH")_2"NH"#, and 2,6-dimethylpyridine.

Bulk is important in determining the axial/equatorial positions of groups in cyclohexane rings.

The t-butyl group will always be equatorial.

When two groups are competing for position, the bulkier group will be equatorial.