L6 - Alkenes and Alkynes

objectives

• construct models, identify the functional group, and write structural and molecular formulae for homologous series of organic chemical compounds, up to C8:
  • alkenes
  • alkynes
• analyse the shape of molecules formed between carbon atoms when a double and triple bond is formed between them

General information about Alkenes

Alkenes are:

• members of a homologous series
• hydrocarbons - contain only C and H
• general formula is C_nH_2n - for non-cyclic alkenes
• unsaturated - atoms can be added to their formula
• contain a C=C double bond somewhere in their structure

General Information about Alkynes

Alkynes:

• Hydrocarbon molecules with one or more triple bonds are called alkynes;
• they make up another series of unsaturated hydrocarbons.
• Two carbon atoms joined by a triple bond are bound together by one σ bond and two π bonds. The sp-hybridized carbons involved in the triple bond have bond angles of 180°, giving these types of bonds a linear, rod-like shape.
• The simplest member of the alkyne series is ethyne, C2H2, commonly called acetylene. The Lewis structure for ethyne, a linear molecule, is:

• The IUPAC nomenclature for alkynes is similar to that for alkenes except that the suffix -yne is used to indicate a triple bond in the chain. For example, CH₃CH₂C≡CH is called 1-butyne.
• General Formula = C_nH_2n-2

Naming Alkenes

Alkenes are named according to standard IUPAC rules

• select the longest chain of C atoms containing the double bond;

• place the ending -ENE on the basic name

• number the chain starting from the end nearer the double bond

• use a number to indicate the lower number carbon of the C=C

• as in alkanes, prefix with substituents

• side chain positions are based on the number allocated to the first C of the C=C

e.g. CH₃ - CH = CH - CH₂ - CH(CH₃) - CH₃ is called 5-methylhex-2-ene

We can get positional isomerism of Alkenes and Alkynes

Molecular Shape of Alkenes and Alkynes

Alkenes

• spatial arrangement around the C=C is planar
• the bond angles are 120°
• This is because the Double bond counts as ONE ELECTRON DOMAIN
• This means there are 3 different electron domains
• Resulting in a TRIGONAL PLANAR Shape

Alkynes

• Spatial arrangement around the C≡C is Linear
• The bond angles are 180°
• This is because the Triple bond counts as ONE ELECTRON DOMAIN
• This means there are 2 different electron domains
• Resulting in a LINEAR Shape