How can you distinguish among single, double and triple convalent bonds?

Question

Luna Chen · Accepted Answer

Organic chemists speak of the #"the degree of unsaturation"#, and this is a very useful rule of thumb to apply to organic formulae.

An alkane, any alkane, has a formula of #C_nH_(2n+2)#; try it with the regular alkanes, methane, ethane, propane........etc. When given a formula, this is the first thing to check. As regards heteroatoms, oxygen in the formula may be ignored; if there is nitrogen, we subtract #NH# from the formula; #"halogens"# count for #"1 hydrogen"#. These basic guidelines allow us to evaluate the composition of many organic formulae. #"Methane, n=1"#, is saturated, so is #"ethane"#, and #"propane"#, and #"butane"#. What are the formulae here? But now we go to ethylene, #H_2C=CH_2#, which clearly does not fit the alkane formula. Because ethylene has a #C_2H_4# rather than the #"saturated"# #C_2H_6# formula, ethylene is said to have #"1 degree of unsaturation"#, that is 2 hydrogens LESS than the saturated formula. #"Propylene"# has a formula of #C_3H_6#; again #1""^@# of unsaturation. So what do we do with the definition? Every #"degree of unsaturation"# specifies a double bond (i.e. #C=O#, #C=N#, #C=C#), OR a ring junction: compare hexane, #C_6H_14#, no degrees of unsaturation, to cyclohexane, #C_6H_12#. The ring junction, the #C-C# bond, has reduced the hydrogen count by 2, so that cyclohexane has the 1 degree of unsaturation. #"Benzene"# has 4 degrees of unsaturation given its cyclic #C_6H_6# formula. Given your question, a formula of #C_nH_(2n+2)O# is saturated, an alcohol or an ether, a formula of #C_nH_(2n)O# is unsaturated, and possesses a #C=C#, OR a #C=O# bond, OR a ring. In order to determine what potential functionality a formula may have, it is always helpful to evaluate its degree of unsaturation. And finally, I'll give some examples. How many degrees of unsaturation are there in #"acetylene"#, #H-C-=C-H#, in #"acetone, "C_3H_6O#, in #"cyclohexanone, "C_6H_10O#, in #"toluene, "C_7H_8#? I apologize if this did not answer your question; I (as well as others) will be happy to hear more.

Wyatt Adkins · Answer

undefined The sharing of one pair of electrons occurs in a single covalent bond, two pairs of electrons occur in a double covalent bond, and three pairs of electrons occur in a triple covalent bond between two atoms.