alkenes Flashcards
how do C=C bonds differ from C-C bonds
- single bonds form from the end on overlap of two S bonds to form a sigma bond, which is free to rotate
- double bonds form from the sideways overlap of two p orbitals to make a pi bond this can not rotate as it would break the overlap of P orbitals
what differs in the strength of a sigma bond compared to a pi bond and what does this man about the bonds making up a C=C
sigma bond is stronger than a pi bond, meaning the two bond in a C=C bond are not the same despite being drawn that way
define stereoisomerism
molecules with the same structural formula but different arrangement of atoms in space.
what is E/Z isomerism and how does it occur
when there are two different groups of atoms bonded to each C in a C=C which is unable to rotate
(1. z on the same side
2. e on different sides)
what features are required for E/Z isomerism to occur
- two different groups attached to the carbons on the double bond so if they were to be rotated then a different molecule would be formed
- restricted rotation - the double bond can not rotate without breaking the pi bond.
what is cis/trans isomerism
when the same group is attached to each of the C in the C=C double bond
(1. cis on the same side
2. trans is on opposite sides)
why would cyclohexene be more reactive than cyclohexane
double bond of cyclohexene has a weak pi bond and a stronger sigma bond, cyclohexane only has a strong sigma bond, it is therefore easier to break the bond of cyclohexene than cyclohexane
what would you see when cyclohexene is added to bromine water
- when first added a clear colorless layer forms above the orange bromine water
- when shaken the orange colour of the bromine rapidly disappears and two colorless layers remain
what would you see when cyclohexane is added to bromine water
- when first added a clear colourless layer will form above the orange bromine water
- when shaken the two layers remain but the orange colour transfers to the organic layer as bromine is more soluble in organic compounds than water
why would the reaction between bromine water and cyclohexane/ cyclohexene commonly happen in the dark
in sunlight the uv light has enough energy for radical substitution of the bromine, leading to a slight decoloring of the bromine water which may make results ambiguous
define addition reaction
a reaction in which a reacted is added to an unsaturated molecule to make a new saturated molecule
define electrophile
an atom or group of atoms which is attracted to an electron rich center where it accepts a pair of electrons to form a new covalent bond
what is the mechanism for the addition to alkenes
electrophilic addition
what is heterolytic fission
when two new ions are formed because both of the electrons go to the same atom.
how can electrophilic addition occur with Br-Br even though it isn’t polar?
When the Br-Br molecule approaches the pi electron cloud, the electrons in the bromine molecule are repelled away from pi bond. This produces an induced dipole with the Br closest to the pi bond slightly positive, and the one further away slightly negative.
