This horizontal trends also tells us that amines are more nucleophilic than alcohols, although both groups commonly act as nucleophiles in both laboratory and biochemical reactions. The reasoning behind the horizontal nucleophilicity trend is the same as the reasoning behind the basicity trend: more electronegative elements hold their electrons more tightly, and are less able to donate them to form a new bond. Moving horizontally across the second row of the table, the trend in nucleophilicity parallels the trend in basicity: The increase in reactivity with increasing atomic number only occurs for the metals in groups 1 and 2. (This is the reverse of the reactivity trend of the metals of groups 1 and 2, in which reactivity increases down a group. There are predictable periodic trends in nucleophilicity. Zinc is the most reactive in group 12, and mercury is the least reactive. Periodic trends and solvent effects in nucleophilicity It should not be surprising, then, that most of the trends in basicity that we have already discussed also apply to nucleophilicity. When thinking about nucleophiles, the first thing to recognize is that, for the most part, the same quality of 'electron-richness' that makes a something nucleophilic also makes it basic: nucleophiles can be bases, and bases can be nucleophiles. Reactions with carbon nucleophiles will be dealt with in chapters 13 and 14, however - in this chapter and the next, we will concentrate on non-carbon nucleophiles. Of course, carbons can also be nucleophiles - otherwise how could new carbon-carbon bonds be formed in the synthesis of large organic molecules like DNA or fatty acids? Enolate ions (section 7.5) are the most common carbon nucleophiles in biochemical reactions, while the cyanide ion (CN -) is just one example of a carbon nucleophile commonly used in the laboratory. When the pressure can be felt, take your thumb off and test the gas with a lighted splint. 2M ( s) + 2H2O ( l) 2MOH ( aq) + H2 ( g) Figure 8.4.1: A small piece of potassium metal explodes as it reacts with water. Drop another bit of magnesium into the first test-tube and put your thumb over the end. 2M ( s) + Cl2 ( g) 2MCl ( s) Alkali metals react with water to produce hydrogen gas and alkali metal hydroxides this is a very exothermic reaction (Figure 8.4.1 ). Compare the reactivity of the two metals. Into the other, drop a small piece of calcium. Moving down Group 1 elements, like cesium and water, shows a clear trend in reactivity. Into one test-tube drop a small piece of magnesium. Trend in Reactivity: Explanation for Increasing Reactivity. This means that when cesium reacts with water, it releases heat into its surroundings. More specifically in laboratory reactions, halide and azide (N 3 -) anions are commonly seen acting as nucleophiles. The net enthalpy change for the cesium-water reaction is exothermic (heat-releasing). In both laboratory and biological organic chemistry, the most relevant nucleophilic atoms are oxygen, nitrogen, and sulfur, and the most common nucleophilic functional groups are water, alcohols, phenols, amines, thiols, and occasionally carboxylates. Nucleophilic functional groups are those which have electron-rich atoms able to donate a pair of electrons to form a new covalent bond.
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