Alkenes Flashcards
3 steps for naming alkenes:-
1) choose end closest to the double bond and start numbering the longest chain.
2) if double bond is between between 2 and 3 write the lowest number.
3) write this number before -ene.
E.g 4-ethyl non-2-ene
What is a sigma bond ?(sometimes shown as squashed-6 = sigma)
P-orbitals overlapping end to end
What is a Pi-bond?
P-orbitals overlap side-to-side (weak bond)
What makes up alkene double bond?
Sigma bond bonds two carbons, carbons have P-orbital visible above and below.
Pi-bond is formed by orbital side to side overlap (looks like two banana bonds around the sigma, between the Cs)
Rotation:-
Free rotation about single bonds.
Lack of rotation about double bonds as would require breaking pi-bond= not enough energy.
What is stereoisomerism?
Compounds with the same structural formula but a different arrangement in space.
Preparation of ethene via catalytic dehydration:-
Ethanol -> ethene + H2O
Cis:-
2 identical groups on same side of C=C
Trans:-
2 identical groups of opposite sides of C=C
Cahn-Ingold-Prelog rules (E/Z)
Prioritise the 2 groups attached to each C of C=C based on atomic number.
Z:-
Top priority (atomic number) groups on same side. “Togezzer”
E:-
Top priority groups on opposite side = E.
Elimination (dehydration) making an alkene:-
Alcohol —> alkene + water
—-> = w/ concentrated H2SO4
Sentence for ethanol making conditions:-
Steaming (steam) drunk at a 60th (pressure) birthday party, 300 people (warm room, 300oC) there drinking vodka (ethanol, alcohol made like this) and coke (contains phosphoric acid/ H3PO4).
Heterolytic fission:-
Bond breaking, both electrons in the covalent bond go to just one of the bonded atoms.
What does a curly arrow show?
Movement of a pair of electrons
What is an electrophile?
Electron pair acceptor
Part 1 of electrophilic addition:-
Alkene with 4 electrons in its double bond is approached by a bromine (Br2) molecule and the bromine’s electrons are repelled to the far end of the bromine molecule.
Part 2 of electrophilic addition:-
Delta positive end of bromine molecule becomes an electrophile and so accepts electron pair from pi-bond. New covalent bond therefore created and so covalent bond in the bromine molecule is broken by heterolytic fission.
Part 3 of electrophilic addition:-
Negative bromide ion formed is attracted to the carbocation (cat ion) left after the pi-bond broke and a new dihalogen alkene is formed.
Markownikoff’s rule:-
When an unsymemetrical alkene reacts with a molecule e.g. HCl (g) the H atom of HCl goes to the double bonded carbon with the most H atoms already.
THE C WITH THE MOST H IN THE ALKENE GETS THE H WHEN H-X IS ADDED.
Showing polymers:-
"n" of a monomer, bracketed = a repeat unit,
Must be two carbons in visible width:-
H H
| |
--C----C--
| |
CH3 CH3
In brackets n
= poly(but-2-ene)
How do you name polymers?
Monomer name in brackets, prefixed by poly e.g. poly(ethene)
Alkene functionsl group:-
\ /
C===C
/ \
