An Introduction to Addition Reactions of Alkenes and Alkynes

PROBLEM 3.26 Calculate the degrees of unsaturation (Q) for the following formulas. Give two possible structures for each one. (a) C5H6 (b) CyHg (c) CiqHio (d) C5HgBr2. 

FIGURE 3.68 Addition of hydrogen chloride to 2,3-dimethyl-2-butene. 

In this reaction, the n bond of the alkene and the C bond of H — Cl have been converted into a pair of C bonds attaching H and Cl to carbon atoms. We must lead into this reaction with a look at the structure and properties of molecules such as hydrogen chloride that contain bonds between two very different kinds of atoms. 

Recall that //bonds between different atoms are polar— the electrons in the bond cannot be shared equally in a covalent bond between different atoms. The limit of this phenomenon is an ionic bond in which two oppositely charged species are held together by the electrostatic attraction between them. Potassium chloride and sodium fluoride are examples (Fig. 3.69). Charges also exist in covalent bonds between 

FIGURE 3.69 Two examples of charge-separated, ionic bonding. 

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