Chapter 13 - Alkenes Flashcards
What is the general formula of an alkene?
CnH2n
How is a π-bond formed?
The sideways overlap of two adjacent p-orbitals above and below the bonding C atoms. Each carbon atom contributes one electron to the electron pair in the π-bond.
The π-bond restricts rotation about the C=C bond.
What is the shape around a double bond?
A trigonal planar
- there are three regions of electron density around each of the carbon atoms
- the three regions repel each other as far apart as possible, so the bond angle around each carbon atom is 120 degrees
What are stereoisomers?
Compounds with the same structural formula but with a different arrangement in space.
What conditions in a molecule are required for E/Z isomerism?
- A C=C double bond, as the π-bond restricts movement of the groups attached to the double bond so the attached groups have fixed positions in space.
- Different groups attached to each carbon atom of the double bond (e.g. but-2-ene can show E/Z isomerism whereas But-1-ene can’t).
What is a special condition required for cis-trans isomerism (a type of E/Z isomerism)?
One of the attached groups on each carbon atom of the double bond must be the SAME.
What is the difference between a cis isomer and a trans isomer?
The cis (Z) isomer has the hydrogen atoms (or higher priority groups) on each carbon in the double bond on the same side of the molecule.
The trans (E) isomer has the hydrogen atoms (or higher priority groups) diagonally opposite each other.
What are the Cahn-Ingold-Prelog rules and how do they work?
Rules used to classify alkenes as E or Z isomers.
In this system the atoms attached to each carbon atom in a double bond are given a priority based upon their atomic number.
- Higher atomic number → Higher priority
- A Z isomer has both higher priority groups on the same side of the C=C bond.
- An E isomer has both higher priority groups on the opposite sides of the C=C bond
Why are alkenes more reactive than alkanes?
Being on the outside of the double bond, the π-electrons are more exposed than the electrons in the σ-bond. A π-bond readily breaks and alkenes undergo addition reactions relatively easily.
How can you test for unsaturation?
Unsaturated hydrocarbons decolourise bromine water.
Alkene + Br2 → Dihaloalkane
(bromine adds across the double bond)
Orange → Colourless
Indicates the presence of a C=C double bond.
Give 4 types of addition reactions of alkenes.
Alkene + Hydrogen (+ nickel catalyst) → alkane
Alkene + Halogens → dihaloalkane
Alkene + Hydrogen halide → haloalkane
Alkene + Steam (+ acid catalyst e.g. H3PO4) → alcohol
What is an electrophile?
An atom or group of atoms that is attached to an electron-rich centre and accepts an electron pair.
An electrophile is usually a positive ion or a molecule containing an atom with a partial positive (δ+) charge.
What is a carbocation?
A molecule containing a positively charged carbon atom.
According to Markownikoff’s rule, which carbocation is the most stable and the major product?
The more alkyl groups attached to the positively charged carbon, the more stable the carbocation is as the positive charge is more spread out over the alkyl groups in the molecule.
E.g.
A secondary carbocation is more stable than a primary carbocation and would therefore be the major product.
2-bromopropane is formed from a secondary carbocation so it is the major product. 1-bromopropane is formed from a primary carbocation so it is the minor product.
Industrial polymerisation is carried out at…
…high temperatures and high pressure using catalysts.
