2}C CH_{2}
| \\ / \ /
H N N
|
Pyrrolidine, C_{4}H_{9}N Pyrridine, C_{5}H_{5}N H

Piperidine, C_{5}H_{11}N

H H
\ / H
C |
/ \ H_{2}C-----C-----CH_{2}
H_{2}C CH_{2} | | |
| | | NH CH_{2}
HC CH | | |
|\ /| or H_{2}C-----C-----CH_{2}
| N | |
| /|\ | H
|/ H \|
H_{2}C-----CH_{2}
Tropane, C_{7}H_{13}N

H H H H
| | | |
C C C C
// \ / \\ // \ / \\
HC C CH HC C CH
| | | | | |
HC C CH HC C N
\\ / \ // \\ / \ //
C N C C
| | |
H H H

Quinoline, C_{9}H_{8}N Isoquinoline, C_{9}H_{8}N

The common alkaloids are distributed in the several groups as follows:

Pyrridine--piperidine group; piperine, coniine, nicotine.
Pyrrolidine group; hygrine and stachydrine.
Tropane group; atropine, hyoscine, cocaine, lupinine.
Quinoline group; quinine, cinchonine, strychnine, brucine.
Isoquinoline group; papaverine, hydrastine, morphine, codeine, berberine.

The composition and properties of the individual alkaloids have been
extensively studied, because of their medicinal uses. As they have no known
metabolic use to the plants which elaborate them, it will not be worth
while to consider all of these investigations in detail here. The following
facts with reference to certain typical members of each group will serve to
illustrate the general constitution and properties of the alkaloids.

=Piperine=, C_{17}H_{19}O_{3