Alkenes Flashcards
why is a cis alkene less stable than a trans alkene
experiences steric strain
2 alkyl groups on the same side of the C atom in the cis alkene, their electron clouds overlap each other and experience repulsion
why are alkenes highly reactive
has presence of electron rich C=C bond that attracts electrophiles
what are electrophiles
electron pair acceptors that are attracted to electron rich sites
are electron deficient
what makes a molecule electron deficient
has a positive charge
partial positive charge
incomplete octet
how are alkenes nucleophiles
able to donate electron pairs to electron deficient atom to form a covalent bond
how is a molecule electron rich
has negative charge, partially negative charge, has lone elctron pair
what are addition reactions
pi bond in the C=C bond is broken, forming 2 sigma bonds and forming 2 C-C bonds
there is a decrease in saturation
original sigma bond in unsaturated C=C bond remains unaffected
what is heterolytic fission
bond is broken in a way that both of the 2 bonding electrons go to one atom
why is the first step in electrophilic addition a slow step
the step requires higher activation energy due to the heterolytic fission
what are unsymmetrical alkenes
one that does not display cis trans isomerism
has more than 2 types of groups attached to the C=C
has 2 of the same group attached to the same C atom
when is a carbocation stable
the greater the number of electron donating alkl groups attached to the positively charged C atom, the greater the dispersal of the positive charge, the more stable the carbocation
how can stereochemistry allow a racemic mixture from an addition product
alkene’s cis and trans alkene react with electrophile to give same carbocation
nucleophile can attack the positively charged C of the trigonal planar carbocation from either side of trigonal plane with equal likelihood
reaction from each side both form enantiomers
since nucleophilic attack on both sides happen with equal likelihood will result in 50:50 ratio of enantiomeres
why are inert solvents used for halogen additions
allows both halogen and the alkene to be dissolved
why is fluorine not used in halogenation
it reacts explosively with alkenes
why is iodine not used in halogenation
reaction is thermodynamically unfavourable
vicinal diodides are unstable and will decompose back to alkene and iodine
how do partial charges arise on Cl2/Br2 during electrophilic addition of halogens in CCl4
pi electron cloud of alkene repels electron cloud of Br2/Cl2, distorting and polarising Br2/Cl2 electrons cloud, allowing partial charges to arise on each atom of Br2/Cl2
relation between reduction and number of C=C bonds
one mole of C=C bond requires one mole of H2 to reduce so amount of H2 reacted per mole of alkene is also the number of C=C bonds present
relation between halogenation and number of C=C bonds
1 mol of Br2 reacts with 1 mole of C=C, so amount of Br2 reacted with one mole of C=C, is equal to number of C=C in the alkene
distinguising between alkenes and alkanes using halogenation
test: add br2 in ccl4 drop wise to each sample in a test tube at room temp
obeservations:
alkane: no rapid decolourisation of orange red br2
alkene: rapid decolorisation of orange red br2
distinguishing between alkanes and alkenes using hot acidic KmNO4
test: add 2 drops of Kmno4 acidified with dilute h2so4 to each sample in a test tube and heat each reaction mixture in a hot water bath
observations
alkenes: decolourisation of purple kmno4 and brown black ppt of mno2 observed
alkanes: no decolourisation of purple kmno4 and no brown black ppt of mno2 observed
distinguishing alkanes and alkenes but in gaseous state
test: bubble a sample of each gase through br2 (aq) in separate test tubes at room tempalkane: alkane: no rapid decolourisation of orange red br2
alkene: rapid decolorisation of orange red br2
distinguishing between a normal alkene and a terminal alkene
test: add 2 drops of KMno4 acidified with dilute h2so4 to each sample in a test tube and heat each reaction mixture in a hot water bath
bubble any gas produced through limewater
observations:
terminal alkene: decolourisation of purple kmno4 and brown black ppt of mno2 observed
white ppt observed in limewater
normal alkene: decolourisation of purple kmno4 and brown black ppt of mno2 observed
no white ppt observed in limewater
