Organic Chemistry Practice Problems and Problem Sets
periodic trends
Rank the following compounds in order of decreasing nucleophilicity. (1 = most nucleophilic)
Nucleophilicity increases with electron density; negatively charged molecules are more nucleophilic than neutral molecules. So HS- is the best nucleophile in this group.
Nucleophilicity also increases with size. This is because the larger an atom, the more polarizable it is. Size increases as we go down the periodic table, so H2S is a better nucleophile than H2O.
MendelSet practice problem # 539 submitted by Matt on July 3, 2011.
Rank the following electrophiles in order of decreasing reactivity with NaN3 in DMF. (1 = most reactive)
This problem is similar to problem 536. The only difference among these compound is their leaving group, so the compound with the best leaving group will undergo substitution reactions most rapidly.
Good leaving groups are stable. Larger ions tend to be more stable than smaller atoms (due to a smaller charge:size ratio. See problem 288), so when going down the periodic table, stability increases. I- is more stable than Br-, which is more stable than Cl-, etc. So I- is the best leaving group, and 2-iodobutane will react the fastest with a nucleophile.
MendelSet practice problem # 538 submitted by Matt on July 3, 2011.
Rank the group of molecules below in in order of decreasing basicity. (1 = most basic)
Explain your reasoning.
The periodic trend for acidity is increasing acid stength as you move from left to right or from up to down on the periodic table, so the trend for basicity will be opposite; amines (which contain nitrogen) are the most basic neutral compounds, and oxygen is most basic than sulfur.
MendelSet practice problem # 308 submitted by Matt on June 7, 2011.
Draw the conjugate base forms of each acid listed below, then rank the acids in order or decreasing acidity (1 = most acidic).
Explain your reasoning.
Electronegativity increases as you travel from left to right along the periodic table, so the fluorine anion is more stable than a negative oxygen, which is more stable than a negative nitrogen, etc.
Because F- is the most stable, it is the weakest base, and its conjugate acid (HF) is the most acidic.
MendelSet practice problem # 286 submitted by Matt on June 6, 2011.