↝ unsaturated hydrocarbon family (contains
at least one C=C double bond)
↝ general formula: CnH2n
The basic names for the alkenes are the same as for the alkanes, but we replace the ‘ane’ ending with an ‘ene’ (for example, ethane becomes ethene).
However, unlike the alkanes, the double bond of the alkenes provides a twist when naming them. Since the double bond is essentially locked into position, we may have isomers (called stereoisomers, since they have the same formula and structural backbone but a different arrangement of atoms in space) in some situations that we will encounter throughout this lesson.
↝ general formula: CnH2n
The basic names for the alkenes are the same as for the alkanes, but we replace the ‘ane’ ending with an ‘ene’ (for example, ethane becomes ethene).
However, unlike the alkanes, the double bond of the alkenes provides a twist when naming them. Since the double bond is essentially locked into position, we may have isomers (called stereoisomers, since they have the same formula and structural backbone but a different arrangement of atoms in space) in some situations that we will encounter throughout this lesson.
Notice that neither ethene nor propene require a number to locate the
double bond, since there is no other place to locate the double bond in
chains with so few carbons.
However, when the chain length increases to four, the double bond could be found in two different locations - either at the 1 carbon (but-1-ene) or the 2 carbon (but-2-ene)*. It also possible to have the cis isomer or the trans isomer to give cis-but-2-ene and trans-but-2-ene, respectively.
To determine whether the isomer is cis or trans, draw a line through the double bond, parallel to its length. If the main chain carbons coming off the double bond are on the same side of the line, it is the cis isomer. However, if the main chain carbons coming off the double bond are on the opposite side of the line, it is the trans isomer.
NameIts!
The method for naming alkenes is basically the same as for alkanes, with the addition of locating the double bond and indicating the cis/trans isomer (when necessary). One last tip ↠ be sure when naming these compounds to find the longest chain that contains the multiple bond and start numbering from the end closest to the double bond (in other words, the double bond takes precedence over everything else).
However, when the chain length increases to four, the double bond could be found in two different locations - either at the 1 carbon (but-1-ene) or the 2 carbon (but-2-ene)*. It also possible to have the cis isomer or the trans isomer to give cis-but-2-ene and trans-but-2-ene, respectively.
To determine whether the isomer is cis or trans, draw a line through the double bond, parallel to its length. If the main chain carbons coming off the double bond are on the same side of the line, it is the cis isomer. However, if the main chain carbons coming off the double bond are on the opposite side of the line, it is the trans isomer.
NameIts!
The method for naming alkenes is basically the same as for alkanes, with the addition of locating the double bond and indicating the cis/trans isomer (when necessary). One last tip ↠ be sure when naming these compounds to find the longest chain that contains the multiple bond and start numbering from the end closest to the double bond (in other words, the double bond takes precedence over everything else).
Here are the answers.
Alkynes
↝ unsaturated hydrocarbon family (contains at least one C𝄘C triple bond)
↝ general formula: CnH2n-2
The names for the alkynes are essentially the same as for the alkanes, but we replace the ‘ane’ with a ‘yne’ (for instance, ethane becomes ethyne).
NameIts!
There are no cis/trans isomers with alkynes because the main chain around the triple bond is linear (180° angle - see in the diagrams below).
Again, when naming these compounds, find the longest chain that contains the multiple bond and start numbering from the end closest to the triple bond.
Here are the answers.
* Note: When I learned organic nomenclature (and for the majority of the years I taught it) the names would have been 1-butene, cis-2-butene and trans-2-butene. Recently, IUPAC revamped the naming by splitting open the alkene name and inserting the number. So, now it is written as but-1-ene, cis-but-2-ene and trans-but-2-ene. The only reason I tell you this is that I spent about 20ish years using the "old" method and I'm bound to revert to it without thinking sometimes.
Homework:
Answers:
