Match each pair of compounds on the left and its corresponding melting points (m
ID: 956538 • Letter: M
Question
Match each pair of compounds on the left and its corresponding melting points (m.p.) or boiling points (b.p.) in degree C to a major reason, given on the right, why the physical property increases. Answers may be repeated. CH_3 CH_2 PHCH_3 (m.p. -160), CH_3 P(CH_3)_2 (m.p. -85) CH_3 N(CH_3)_2 (m.p. -117), CH_3 P(CH_3)_2 (m.p. -85) CH_3 CH_2 NHCH_3 (b.p. 36), CH_3 (CH_2)_2 NH_2 (b.p. 48) (CH_3)_2 CHNH_2 (b.p. 33), CH_3 CH_2 NHCH_3 (b.p. 36) CH_3 (CH_2)_2 NH_2 (b.p. 48), CH_3 (CH_2)_2 OH (b.p. 97)Explanation / Answer
1. In this case, the m.p. is higher due to more rotational symmetry. More rotational symmetry leads to more compactness and hence, better stacking of the molecules in the crystal space. Also, another point may be the repulsive forces operating due to the three bulky methyl groups (the secondary phosphine has only alkyl groups) and these two factors combine to increase the melting point of the tertiary phosphine.
2. Here, the melting point of the phosphine is higher due to the higher molecular weight of the phosphine as compared to the amine (the amine has molecular mass 59 a.m.u and the phosphine has molecular mass 76 a.m.u). Higher molecular mass leads to higher amount of heat energy being absorbed by the molecule for stretching, rotation, etc. vibrations and hence, higher melting point.
3. This example is a classic case of hydrogen bonds leading to higher boiling points. Both the amines can engage in hydrogen bonding, but the secondary amine (b.p. 36) has only one hydrogen atom available to participate in hydrogen bonding whereas the primary amine (b.p. 48) has two. Greater number of available hydrogen atoms means greater possibility of hydrogen bonding and hence, higher boiling point.
4. In this case, the primary amine is more compact and has a more sphere-like structure. However, as the compactness increases, the Vander Waal's interactions also decrease leading to a decrease in boiling point.
5. In this case, the higher boiling point of the alcohol (b.p. 97) is attributed to the greater polarity of the O-H bond as compared to that of N-H bond. This polarity in turn leads to greater chances of hydrogen bonding in the alcohol and hence accounts for the higher boiling point of the alcohol.