Alkenes & Alkynes
Previous page:Alkanes

Multiple bonds

Some times a couple neighbouring carbons will, for a variety of reasons, shed a hydrogen each and form a second bond between them.

 

 

H
H
|
|
C
=
C
|
|
H
H
H
H
H
H
H
|
|
|
|
|
H
-
C
-
C
-
C
=
C
-
C
-
C
-
H
|
|
|
|
|
H
H
H
H
H

Electron dot diagram and expanded structural formula for ethene. 
Hex-3-ene on right.

 

 

Obviously methane can’t do this by itself, but ethane becomes ethene (C2H4), propane becomes propene (C3H6) and butane becomes butene (C4H8). Notice both the revised general molecular formula (CnH2n) and the common suffix for these alkenes.

Problem 3. Draw as many versions of heptene (C7H14) as you can.  (If you get more than three you are repeating yourself.)

 

Note, too that a long enough chain may have multiple double bonds.

  

 CH2=CH-CH2-CH=CH2

Structural formula for and model of pent-1,4-diene.

While such a molecule might not fit the alkene formula any longer, it is still an alkene, because the definition of an alkene is not based on its formula, but its structure; possessing one or more double bonds.

Propene, as has been stated, has the formula C3H6, whereas propane is C3H8. A third compound, cyclopropane, has the same formula as propene, without possessing a double bond. As the name suggests, cyclopropane is a three carbon chain, but the end carbons are bonded to each other, making a loop, or cycle.  With no double bonds, cyclopropane in an alkane.  

 
H
H
H
H
\
/
\
/
C
-
-
-
C
\
/
C
/
\
H
H

Structural formula for cyclopropane.

 

 

In some chains triple bonds are possible. Ethyne (commonly known as acetylene) has the formula C2H2. Propyne is C3H4. Pentyne must be C5H8. All alkynes possess at least one triple bond. Their general molecular formula is CnH(n-2).

HCCH

H-C≡C-H

Electron dot diagram and condensed and expanded structural formulae for ethyne.

 

 

Next page: Naming Alkanes etc.

Link to Index