CHAPTER 3: Question 1

3D explanations

A molecule will contain a stereocentre if it contains either:

An atom with four groups, tetrahedrally arranged around the atom, and all four groups are different to one another;

An atom with six groups octahedrally arranged around the atom, with at least three different groups and no more than two of each type.

None of the molecules in parts a-n contain an atom with six groups attached to it, so we only need to be concerned with the first case. It was shown in Chapter 1, that whenever a carbon atom is attached to four different groups by four s-bonds, then the geometry of the groups around that carbon atom will be tetrahedral. Bearing this in mind, each example can now be examined.

a) The molecule does contain an atom (the carbon atom) with four groups tetrahedrally arranged around it as shown below. However, the four groups are not all different (there are two identical chlorine atoms and two identical hydrogen atoms), so the carbon atom is not a stereocentre, and the molecule does not contain a stereocentre.

b) The molecule does contain an atom (the carbon atom) with four groups tetrahedrally arranged around it as shown below. However, the four groups are not all different (there are three identical bromine atoms), so the carbon atom is not a stereocentre, and the molecule does not contain a stereocentre.

c) The molecule does contain an atom (the carbon atom) with four groups tetrahedrally arranged around it as shown below. The four groups are all different (as shown by the different colours below), so the carbon atom is a stereocentre, and the molecule does contain a stereocentre.

d) The molecule does contain an atom (the carbon atom of the CH₃ group) with four groups tetrahedrally arranged around it as shown below. However, the four groups are not all different (there are three identical hydrogen atoms), so the carbon atom is not a stereocentre, and the molecule does not contain a stereocentre. Note that the other carbon atom is also not a stereocentre since it has a trigonal planar geometry, and has only three different groups around it.

e) This molecule contains three atoms (the three carbon atoms) with four groups tetrahedrally arranged around them as shown below. The carbon atom on the left however, is attached to three identical hydrogen atoms and so is not a stereocentre. Similarly, the carbon atom on the right is attached to two identical hydrogen atoms and so is not a stereocentre. However, the four groups attached to the central carbon atom are all different (as shown by the different colours below), so this carbon atom is a stereocentre, and the molecule does contain a stereocentre. Note that in this case, two of the atoms attached to the stereocentre are identical (carbon atoms), however, one of the carbon atoms is part of a CH₃ group whilst the other is part of a CH₂OH group, so the two groups are different to one another. This illustrates that it is important to examine the whole group attached to the possible stereocentre, not just the first atom in the group.

f) This molecule contains two atoms (the two carbon atoms with hydrogen atoms attached to them) with four groups tetrahedrally arranged around them as shown below. However, the four groups attached to the carbon atom of the CH₃ group are not all different (there are three identical hydrogen atoms shown in red below), so this carbon atom is not a stereocentre. Similarly, the central carbon atom is not a stereocentre since two of the substituents are identical (CO₂H groups shown in blue below). Hence, the molecule does not contain a stereocentre.

g) This molecule contains three atoms (the three carbon atoms with hydrogen atoms attached to them) with four groups tetrahedrally arranged around them as shown below. However, the four groups attached to the carbon atom of the CH₃ groups are not all different (there are three identical hydrogen atoms), so these carbon atoms are not a stereocentre. The central carbon atom however is attached to four different groups (as shown by the different colours below) and so is a stereocentre. Hence, the molecule does contain a stereocentre. Note that again, three of the groups attached to the stereocentre start with the same atom (a carbon atom), but they differ elsewhere in the group and this is sufficient to make them different.

h) The carbon atom of the CH₃ group is not a stereocentre since three of the groups attached to it (the hydrogen atoms) are identical. Similarly, the carbon atom of the CH₂D group is not a stereocentre since two of the groups attached to it (the hydrogen atoms) are identical. However, the remaining carbon atom is a stereocentre since it has a tetrahedral geometry and is attached to four different groups as shown below. Hence, the molecule does contain a stereocentre. Note that in this case, two of the groups attached to the stereocentre differ only due to isotopic labelling (a hydrogen in the CH₃ group has been replaced by a deuterium atom in the CH₂D group), but this is sufficient to make the two groups different.

i) Two of the carbon atoms in this molecule have a tetrahedral geometry, these are the two carbon atoms with hydrogen atoms attached to them. The carbon atom of the CH₂OH group is not a stereocentre since two of the groups to which it is attached (the two hydrogen atoms) are identical. The other carbon atom however is attached to four different groups (as shown below) and so is a stereocentre. Hence, the molecule does contain a stereocentre. Note that the nitrogen atom is also tetrahedrally attached to four groups (if the lone pair of electrons is included) but is not a stereocentre since two of the groups (the two hydrogen atoms) are identical.

j) Every carbon atom in this molecule is tetrahedrally bound to four groups. However, with the exception of the carbon highlighted in the diagram below, the carbon atoms are all attached to two or three identical hydrogen atoms and so are not stereocentres. The carbon atom highlighted below however is attached to four different groups (as shown by the different colours) and so is a stereocentre. Hence, the molecule does contain a stereocentre.

k) Four of the carbon atoms in this molecule ate tetrahedrally bound to four other groups. Two of these are the carbon atoms of the CH₃ groups which are not stereocentres since three of the groups to which they are attached (the three hydrogen atoms) are identical. The other two are the top and bottom atoms of the cyclohexadiene ring. Again, these are not stereocentres since two of the groups to which they are attached (the left and right sides of the cyclohexadiene ring) are identical. Hence, this molecule does not contain a stereocentre.

l) This molecule contains seven carbon atoms which are tetrahedrally bound to four groups. However, for all but the carbon atom highlighted below, either two or three of the groups attached to the carbon atom are identical and so the carbon atom is not a stereocentre. In the case of the carbon atom highlighted in the diagram below however, all four groups attached to the carbon atom are different, so this carbon atom is a stereocentre. Hence, the molecule does contain a stereocentre.

m) Both carbon atoms attached to hydrogen atoms in this molecule are tetrahedrally attached to four groups. In the case of the carbon atom of the CH₃ group however, three of these groups (the three hydrogen atoms) are identical, so this carbon atom is not a stereocentre. The other carbon atom however, is attached to four different groups as shown below and so is a stereocentre. Hence, the molecule does contain a stereocentre.

n) Every carbon atom in this molecule has a tetrahedral geometry. However, the carbon atoms of the two CH₃ groups are attached to three identical hydrogen atoms and so are not stereocentres. The remaining carbon atom however is a stereocentre since it is attached to four different groups. The four different groups in this case are two isotopes (H and D) of hydrogen, and two isotopes (¹²C and ¹³C) of carbon, but this is sufficient to make the groups different. Hence, the molecule does contain a stereocentre.

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