IUPAC Name, Structure and Formula of Straight-Chain Alkynes Chemistry Tutorial
- Alkynes are hydrocarbons in which there is a triple bond between two carbon atoms(1):
- The systematic IUPAC(2) name of all simple straight-chain alkynes ends in "yne".
- The systematic IUPAC name of a straight-chain alk-n-yne is made up of three parts(3):
(i) A prefix which tells us how many carbon atoms are in the chain (alk).
1 2 3 4 5 6 7 8 9 10 Prefix: meth eth prop but pent hex hept oct non dec
(ii) An infix which is a number that tells us the location of the triple bond (-n-)
(ii) The suffix yne telling us that a triple bond is present within the chain.
- Note that the preferred IUPAC name(4) may not be the same as the systematic IUPAC name:
molecular formula structure systematic IUPAC name trivial name Preferred IUPAC Name C2H2 H-C ≡ C-H ethyne acetylene acetylene
IUPAC Naming of Straight-Chain Alkynes:
- Identify the longest carbon chain containing the triple bond.
- Determine the prefix for the name of the alkyne based on the number of carbon atoms in the chain.
- Number each carbon atom along the longest carbon chain so that the triple bonded carbon atoms have the lowest possible number.
- Determine the infix for the name of the alkyne based on the location of the triple bond (use the lowest number out of the two carbon atoms joined by the triple bond).
Note that the infix may not be required if the longest carbon chain contains only two or three carbon atoms.
- Determine the suffix for the name of the alkyne. All straight chain alkynes containing one triple bond will end in "yne".
- Write the name for the alkyne in the form of prefix-infix-suffix
Drawing the structure(5) of straight-chain alkynes:
- Break the systematic IUPAC name of the alkyne into its three parts:
alk -n- yne prefix infix suffix
- Determine the number of carbon atoms in the longest carbon chain using the prefix.
- Draw a chain of carbon atoms of the required length using dashes to represent a single covalent bond between each pair of carbon atoms.
- Number the carbon atoms in the chain from left to right.
- Determine the location of the triple bond using the infix.
We know there is a triple bond because the molecule's name has the suffix yne.
- Draw a second and then a third dash, a second and third covalent bond, between the carbon with the same number as the infix, and the carbon atom with a number equal to the infix + 1 (that is, the adjacent carbon atom on the right hand side if you have numbered your carbon chain from left to right).
- Draw dashes around each carbon atom in the chain such that each carbon atom makes 4 bonds.
Note that the carbon atoms involved in the triple bond have already used 3 out of the 4 possible bonds in making the triple bond!
- Complete the structure by placing a hydrogen atom (H) at the end of all the vacant dashes.
Molecular formula of straight-chain alkynes:(6)
- Draw the structure of the straight-chain alkyne.
- Write a skeletal molecular formula using the symbols for carbon and hydrogen:
- Count the number of carbon atoms in total in the structure.
total number of carbon atoms = n
- Write the number of carbon atoms as a subscript number to the right of the symbol for carbon
- Count the number of hydrogen atoms in total in the structure.
total number of hydrogen atoms = y
- Write the number of hydrogen atoms as a subscript number to the right of the symbol for carbon
- The general molecular formula for a straight-chain alkyne is CnH2n-2
where n = number of carbon atoms in the carbon chain