Chemistry - A Molecular Science

Chapter 13 Organic Chemistry

(a)

(b)

phosphate on backbone

Figure 13.40 Space-filling (a) and ball-and-sti

ck (b) models of a short piece of DNA

Color code Phosphorus = orange; Oxygen = red;

Nitrogen = dark blue; Hydrogen = light blue;

carbon = grey. The double helix is shown as a ladder in (b). The sides of the ladder (ribbon) show the phosphate backbone, while the rungs represent one base of each base pair. Hydrogen bonding between rungs is responsible for the double helix structure. The arrows point to identical phosphates on the backbone.

13.7

CHAPTER SUMMARY AND OBJECTIVES Organic chemistry is the study of carbon-b

ased compounds. The two simplest classes of

organic compounds are the alkanes and the alkenes. •

Alkanes contain only sp

3 hybridized carbon atoms and sigma bonds. They are not very

reactive. The names of all alkanes end in –

ane.

The positions of any alkyl groups are

indicated by the number of the carbon atom in the longest continuous chain to which they are attached. Thus, 2-methylpentane has a five carbon continuous chain with a methyl group attached to the second carbon.

(^) •
Alkenes contain carbon-carbon double bonds. The
electrons of the double bond make π
alkenes weak Lewis bases. They react with hy
drohalic acids to produce alkyl halides. Their
names end in –ene. The position of the double bond is given as the atom number of the atom in the double bond with the smallest number. For example, 2-butene is a four-carbon alkene with the double bond between the second and third carbon atoms.
Organic compounds with identical formulas can exist as isomers. •
Constitutional isomers differ in the connectivity of the atoms in the molecule. There are five constitutional isomers of hexane, C
H 6

. 14
-^

Stereoisomers have the same connectivity, but the atoms have different geometric arrangements. Geometric isomers can be fo

und in compounds containing double bonds and

can be either

cis

or

trans

. Enantiomers are non-superimposable mirror images of one another.

Enantiomers must contain a stereocenter, a carbon with four

different

groups attached to it.

The physiological activity of the enantiomers can be very different.

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