Alkenes Flashcards
Alkenes definition.
Unsaturated hydrocarbons with:
-C=C bond
-Containing a pi bond
-And a lowercase sigma bond
-With restricted rotation of the pi bond.
Pi bond definition.
Sideways overlap of adjacent p-orbitals above and below the bonding C atoms- one from each carbon atom of the double bond, one electron from each carbon atom to the electron pair.
Lowercase sigma bond definition.
An overlap of orbitals directly between the bonding atoms, 3 of the 4 electrons used in 3 bonds - 1 to other carbon atom of double bond and 2 to 2 other atoms.
Shape around a double bond.
Trigonal planar.
Double bond shape explanation.
-3 regions of electron density around each of the carbon atoms.
-These 3 regions repel each other as far out as possible.
-So bond angle is 120 degrees.
-All the atoms are in the same plane.
-Central atom bonded to 3 other atoms.
Stereo-isomers definition.
Compounds with the same structural formula but have different arrangements of atoms in space.
E/Z isomerism definition.
- Only occurs in compounds with C=C double bonds.
-An example of stereo isomerism.
-Rotation around the double bond is restricted. - 2 different Groups attached to each carbon atom are therefore fixed relative to each other.
-Because of pi bonds electron density position above and below the plane of the sigma bond.
Conditions for E/Z isomers.
- A C=C double bond.
-Different groups attached to each carbon atom of the double bond.
Cis-trans isomerism definition.
-2 of the substituent groups attached to each carbon atom of the C=C group are the same.
- Same conditions as E/Z- is a special case of them.
-One of the attached groups on each carbon atom must be the same.
Cahn-Ingold-Prelog nomenclature rules.
-Atoms attached to each carbon atom in a double bond are given a priority based on on atomic number.
-If groups of high priority are on same side of double bond, compound is a Z isomer.
-If groups of high priority are diagonally placed across the double bond, compound is E isomer.
E/Z assigning priority rules.
Higher atomic number=Higher priority.
For atoms attached directly to the carbon atoms of the double bond.
Cis-trans nomenclature.
Where there is a H atom on each of the double bonded C atoms in cis-trans isomerism:
-Cis isomer is the Z isomer.
-Trans isomer is the E isomer.
Point of difference nomenclature.
If 2 atoms attached to a carbon atom in the double bond are the same then need to find point of difference.
-Group with higher atomic number at the first point of difference is given the higher priority.
General alkenes reactivity.
-More reactive than alkanes
-because of pi bond
-electrons are on outside of the pi bond so more exposed than in sigma bond
-so pi bond readily breaks and undergo addition reactions quite easily
-low bond enthalpy for pi bond(c=c minus c-c to get enthalpy)
-265kjmol.
Addition reaction w/ hydrogen.
alkene+hydrogen w/ nickel catalyst
423K
forms an alkane
hydrogen added across double bond
called hydrogenation
all double bonds in the alkene are hydrogentaed so may require more hydrogen molecules.
Addition reactions w/ halogens.
room temperature
bromine,chlorine
halogen attaches over double bond like H
form dihaloalkanes
Test for unsaturation in a carbon chain(c=c).
Bromine water added dropwise to alkene sample
bromine adds across double bond
Orange colour disappears=double bond present
saturated=no double bond=no addition reaction=no colour change
C=C will always decolourise bromine water.
Addition reaction w/ hydrogen halides.
gaseous hydrogen halides
room temperature
forms haloalkanes
if alkene liquid, hydrogen halide bubbled through
if gas, both gases are mixed
can react w/ conc HCL or hydrobromic acids(H halide sols in water)
Unsymmetrical alkene+ unsymmetrical compound–> 2 possible products can be formed(each carbon of double bond switch)
Addition reactions w/ steam(hydration reaction).
alkenes+steam–>alcohols
in presence of phosphoric acid catalyst, H3PO4
steam adds across double bond(one OH and 1H)
ethanol from ethene in industry
2 possible products also.
Electrophile definition.
An electron pair acceptor.
Attracted to pi electron high density area.
Mechanism for alkenes to form saturated compounds is electrophilic addition.
Positive ion/molecule containing atom w/ delta + charge.
Electrophilic addition mechanism.
Practice drawing it out.
Electrophilic addition mechanism explanation(propene,bromine).
Br non-polar
Pi electrons interact w/ Br-Br bond
Bond polarises
One end becomes delta + and one delta minus(induced dipole)
Electron pair in pi bond attracted to delta + Br
Double bond breaks
Bond formed between one C atom from double bond and one Br atom
Br-Br breaks by heterolytic fission
E pair goes to delta - end of molecule
Br- ion and a carbocation formed
Br- reacts w/ carbocation to form addition product
Induced dipole formation.
Electrons evenly distributed on Br to begin with between two atoms of Br
As Br approaches negative pi bond, e in br bond are repelled
Then end closer to pi bond=+ and minus other end(delta).
Markownikoff’s rule.
When an acid(Hx)(H halide) is added to asymmetric alkene, acidic H attaches itself to the carbon w/ the greater number of hydrogens/bonded to the most H atoms already.
