alkenes Flashcards
what are the physical properties of alkenes
- increasing bp and mp with increasing number of carbon atoms as there are more electrons and stronger id-id interactions as a result
- branched alkenes are more spherical and have less SA of contact between molecules for electron interactions so it has weaker id-id int
- cis has higher bp due to it having a larger net dipole moment causing it to have stronger pd-pd interactions which require more energy to overcome
- trans has higher mp due to the close packing of trans molecules
- insoluble in polar solvents
How to form alkenes
From alc or halogenoalkanes
1. excess conc H2SO4 and heat
2. NaOH or KOH in ethanol and heat
how to identify major product for dehalogenation of halogenoalkanes (elimination)
- major has more alkyl grps bonded to the C atoms in the C double bond C bond
what type of reactions do alkenes undergo and why
electrophilic addition
- C double bond C in alkenes contain a strong sigma bond and weak pi bond
- pi bond serves as a source of electrons/region of high electron density
- makes alkenes vulnerable to electrophilic attack by electrophiles (positive or partial positively charged)
explain electrophilic addition
- 2 step process
1. Form carbocation by EA of H delta+ from polar H-X molecule forming X- showing two electrons
2. attack on carbocation by X- drawing arrow from electrons to the C with a positive charge
how to determine major product during EA
Use markovnikovs rule - whichever produces a more stable carbocation meaning whichever has more alkyl grps bonded to the positively charged C
during addition of Br2 in CCl 4 although there is no net dipole moment why does Br- form
- electron cloud of pi bond in ethene polarises the approaching Br2 molecule causing partially positive Br molecule to attack ethene
how to test for alkenes
decolourisation of orange-red bromine in CCl4 solvent or orange Br2 in H2O
