13 Alkenes Flashcards
What is a double bond made up of?
A sigma bond and a pi bond.
How is a pi bond formed?
A pi bond is formed by the sideways overlap of two p orbitals, one from each carbon atom of the double bond.
Why are pi bonds weaker than sigma 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 pi bonds have a relatively low bond enthalpy.
What is the shape around a double bond?
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 is E/Z isomerism?
A type of stereoisomerism in which different groups attached to each carbon of a C-C double bond may be arranged differently in space because of the restricted rotation of the C-C bond.
What two conditions does a molecule have to satisfy to have E/Z isomerism?
1) A C-C double bond
2) Different groups attached to each carbon atom of the double bond.
What is cis(Z) isomerism?
It has H atoms on each carbon on the same side.
What is trans(E) isomerism?
It has H atoms in each carbon on different sides - diagonally opposite each other.
What are the Cahn-Ingold-Prelog rules?
- If the groups of higher priority are on the same side of the double bond, the compound is the Z isomer.
- If the groups of higher priority aware diagonally placed across the double bond, the compound is the E-isomer.
How do you assign priority?
The higher the atomic number, the higher the priority.
What is the mechanism for the reaction between but-2-ene and hydrogen bromide?
1) Bromine is more electronegative than hydrogen, so hydrogen bromide is polar and contains the dipole H-Br. (sigma+&-)
2) The electron pair is the pi bond is attracted to the partially positive hydrogen atom, causing the double bond to break.
3) A bond forms between the hydrogen atom of the H-Br molecule and a carbon atom that was part of the double bond.
4) The H-Br bond breaks by heterolytic fission, with the electron pair going to the bromine atom.
5) A bromide ion and a carbocation are formed.
6) The Br- ion reacts with the carbocation to form the addition product.
What is an electrophile?
An atom which is attracted to an electron-rich centre of an atom, where it accepts a pair of electrons to form a new covalent bond.
What are stereoisomers?
Compounds with the same structural formula but with a different arrangement of the atoms in space.
What type of addition reactions can alkenes undergo?
- Hydrogen in the presence of a nickel catalyst
- Halogens
- Hydrogen halides
- Steam in the presence of an acid catalyst.
What is the mechanism between propene and bromine?
When bromine approaches an alkene, the pi electrons interact with the electrons in the Br-Br bond.
1) This interaction causes polarisation of the Br-Br bond, with one end of the molecule becoming Br sigma +, and the other end of the molecule becoming Br sigma -. This is known as an induced dipole.
2) The electron pair in the pi bond is attracted to the bromine sigma positive end of the molecule, causing the double bond to break.
3) A bond has now been formed between one of the carbon atoms from the double bond and a bromine atom.
4) The Br-Br bond breaks by heterolytic fission, with the electron pair going to the bromine negative end of the molecule.
5) A bromide ion (Br-) and a carbocation a formed.
6) In the final stage of the reaction mechanism the Br- ion reacts with the carbocation to form the addition product of the reaction.
