Chemistry Reference
In-Depth Information
D One way to explain these facts is to use the formula equivalence between unsaturated
and cyclic structures. It is very useful to know how many of these double bond
equivalents (DBE) are present in a molecular formula. So far, in terms of the general
formulae for hydrocarbons, this has been simple. What happens when the formula has
heteroatoms in it?
The following simple equation gives the number of multiple bonds and/or rings in
a molecular formula. This can be used for all neutral compounds that have carbon,
hydrogen, nitrogen, oxygen, or halogens.
DBE =
1
/
2
[
]
2n
4
+
n
3
−
n
1
+
2
n
4
=tetravalentatoms(carbons)
n
3
=trivalentatoms(nitrogens)
n
1
=monovalentatoms(hydrogens,halogens)
Try this equation on the formula C
10
H
16
NO
3
Br.
E Your calculation should have given a DBE = 3. This answer shows that the molecule
must have one of the following: 3
π
bonds; 2
π
bonds and 1 ring; 1
π
bond and 2 rings;
3 rings.
The DBE might not seem very useful by itself, but it is usually used with other struc-
tural information such as functional groups. Whenever you suggest a structure for any
molecule, it is useful to check if the DBE fits your structure.
Q 3.1.
Give named structures that meet the following descriptions:
(a) Five structural isomers with formula C
6
H
14
.
(b) Four structural isomers with formula C
4
H
8
.
(c) Six structural isomers of the alcohols with formula C
4
H
8
O.
(d) Four structural isomers of the aldehydes with formula C
4
H
6
O.
(e) Four structural isomers of the 1° or 2° amides with formula
C
4
H
9
NO.
Q 3.2.
Draw sawhorse diagrams of the two different staggered and two
different eclipsed conformers of butane that are caused by rotation
around the C
2
-C
3
bond.
Q 3.3.
Which of the conformational isomers in
Q 3.2
is the most stable
(lowest energy). Why?
Q 3.4.
Which of the following alkenes can have
cis
/
trans
stereoisomerism?
Where
cis
/
trans
isomers are possible, draw named structures for the
isomers.
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