Alkenes (page 96) Flashcards
What are Alkenes?
Alkenes are unsaturated Hydrocarbons
(an alkene is like an alkane’s wild younger brother. They look similar, but alkenes are way more reactive)
What is the general formula for Alkenes?
CnH2n
They’re made of carbon and ghydrogen atoms, so they’re hydrocarbons.
look at examples of alkenes, propens, buta ,3-diene, and cyclopenteire on diagram 1 on page 96.
Alkene molecules all have at lease one C=C double covalent bond. Explain what this means?
Molecules with C=C double bonds are unsaturated because they can make more bonds with extra atoms in addition reactions.
What is a ‘Double Bond’ made up of?
Sigma (σ) Bond and a Pi (π) Bond
When is a σ (sigma bond) formed?
when two S orbitals overlap. (see diagram 2 on page 96)
The two S orbitals overlap in a straight line - this gives the highest possible electron density between the two nuclei. This is a single covalent bond.
The high electron density between the nuclei means there is a strong electrostatic attraction between the nuclei and the shared pair of electrions. This means that σ bonds have a high bond enthaply - they are the strongest type of a covalent bonds.
The C-C and C-H bonds in alkanes are what bonds?
they are all sigma bonds.
A π (pi) bond is formed by the sideways overlap of the two adjacent p orbitals, why?
it’s got two parts to it - one ‘above’ and one ‘below’ the molecular axis. This is because the p orbitals which overlap are bumb-bell shaped. (see diagram 3 on page 96).
π bonds are much weaker than σ bonds because the electron density is spread out above and below the nuclei. This means that the electrostatic attraction between the nuclei and the shared pair of electrons is weaker, so π bonds have a relatively low bond enthalpy (see page 40 for more on orbitals).
(enthalpy describes the amount of energy stored in a bond between atoms in a molecule)
Alkenes are much more reactive than alkanes, why?
Alkanes only contain C-C and C-H σ bonds, which have a high bond enthalpy and so are difficult to break. The bonds are also non-polar so they don’t attract nucleophiles or electrophiles. This means alkanes don’t react easily.
Alkenes are more reactive than alkanes because the C=C bond contains both a σ bond and a π bond.
The C=C double bond contains four electrons so it has a high electron density and the π bond is likely to be attacked to electrophiles (see page 100). The low bond enthalpy of the π bond also contributes to the reactivity of alkenes.
Because the double bonds so reactive, alkenes are handy starting points for making other organic compounds and for making petrochemicals.
see diagram 4 on page 96)
(each doulbe bond is like a hot dog. the π bond is the bun and the σ bond is the middle like the saugage).
Consider the hydrocarbons ethane and ethene
a) Explain how the type of bonding differs in these two molecules (2marks)
b) Explain which of these molecules is more reactive (2 marks)
a) Ethane is an alkane, so has a single C-C bond made up of a σ bond (1 mark). Ethene is an alkene, so has a C=C double bond made up of a σ bond and a π bond [1 mark]
b) Ethene will be more reactive than ethane because the double bond has a high electron density/ the π bond sticks out above and below the plane of the molecule, so it attracts electrophiles [1 mark] and because the π bond has alow bond enthalpy so is more easily broken than the C-C σ bond in ethane [1 mark].
(doulbe bonds are always made up of a σ bond and a π bond. So even though π bonds are weaker than σ bonds, double bonds will be stronger than single bonds because they have combined strength of a σ and a π bond).
