Alkenes Flashcards
what are alkenes physical state
2-4 C atoms are gases
5-14 C atoms are liquids
>14 C atoms are solids
characteristics of alkenes
less dense than water
insoluble in water
bonding in C2H4
- each C atom of the double bond is sp2 hybridized (has 3 sp2 hybrid orbitals and 1 unhybridized p orbital)
- each C-H bond is a sigma bond formed by the overlapping of sp2 hybrid orbital of carbon and 1s orbital in H
- one C=C bond is a sigma bond formed by the overlapping of sp2 hybrid orbitals of the C atom
- the unhybridized p orbitals then overlap sideways to give rise to a pi molecular orbital (pi bond is a weak bond)
structure of ethene
molecule is planar (all the carbon atoms lie on the same plane)
H-C-H is 120 degrees
origin of cis-trans isomerism
free rotation of atoms or groups of atoms can take place around a C-C single bond since the sigma electrons of the sigma bond are not in the pathway of the rotation
free rotation cannot take place about a C=C since the pi electrons will be in the pathway of the rotation
the lack of the free rotation is responsible for forming cis-trans isomerism
reason for electrophilic addition reactions
the C atoms of the double bond have available pi electrons
electrophiles will be attracted to these C atoms
since the C atoms of the double bond are unsaturated, the electrophiles add to the double bonded C atoms, hence the alkenes take part in electrophilic addition reactions
what are electrophiles
particles which are looking for electrons
conditions for electrophilic addition
done by dissolving Br in CCl4 (a non-polar solvent in this reaction) and passing ethene through the mixture
there is a colour change from reddish-brown to colourless
process of electrophilic addition
- As the Br molecule approaches the C2H4 molecule, the pi electrons of the double bond polarizes the Br molecule to make one of them partially +ve
- partially +ve Br atom begins to bond with the carbon atom and forms an unstable +vely charged intermediate (carbocation)
- unstable carbocation combines with -vely charged Br to complete the reaction and form a dative covalent bond
addition of hydrogen halides
alkenes react readily with hydrogen halides
HX adds across the double bond
Markovikov rule
whenever HX is added to an unsymmetric alkene, the H always bond to the C atom which has the bigger number of H atoms
reaction with aqueous Br
when an alkene is reacted with aqueous Br, a mixture of 3 products can be formed
due to Br reacting with water, forming HBr an HOBr
as a result there is addition of:
Br-Br (C2H4Br2)
H-Br (C2HBr)
HO-Br (C2H4OHBr)
hydrogenation
when an alkene is reacted with H2, the H atoms add across the double bond and converts the alkene to an alkane
the reaction takes place at140-200 degrees C in the presence of a Ni catalyst
reaction with H-OH (steam)
takes place by mixing the alkene with steam at a pressure of 70atm and passed over a catalyst of H3PO4 at 300 degree C
2 types of oxidation of alkenes
- mild oxidation
- strong oxidation
