Alkenes Flashcards
What is an orbital overlap?
a singly occupied orbital on one atom overlaps with a singly occupied orbital on another atom
What is a sigma bond?
2 p-orbitals each with 1 electron overlap end to end
What is a pi bond?
2 p-orbitals each with 1 electron overlapping sideways
What shape are alkenes?
trigonal planar (120’) around each c
Why are alkenes rigid?
rotation is not possible around the double bond as it would destroy the pi bond
Why are alkenes more reactive than alkanes?
pi bonds are weaker than sigma bonds
What are alkenes?
unsaturated hydrocarbons that form a homolugus series
What is the alkene general formula?
CnH2n
What are stereoisomers?
compounds with the same formular but different arrangement in space
What are the CIP rules for naming stereoisomers?
assign each of the two groups attached to 1 carbon priority - highest atomic no of atome directly bonded to carbon has priority
if they have the same atomic no. look at the next atom
What is a Z isomer?
priority groups on the same side of the C=C
What is an E isomer?
priority groups on the opposite sides of the C=C
What is the Cis/trans system?
the C=C must be attached to two different groups and 1 of the attached groups must be hydrogen
What is a Cis isomer?
two identical groups on the same side of the C=C
What is a trans isomer?
two identical groups on opposite sides of the C=C
What type of reaction is dehydration of alcohols?
elimination = water is removed
Where does the double bond form after dehydration of an alcohol?
forms next to where the OH was removed from, there may be two possible products
What is Markowinkoff’s rule?
the major product of the addition reaction will be the one formed via the more stable carbocation intermediate. The stability is in the order primary < secondary < tertiary
What is electrophyllic addition?
an addition reaction where a chemical compound containing a double or triple bond has a π bond broken, with the formation of two new σ bonds.
What do curly arrows show in a reaction mechanism?
represent the movements of a pair of electrons
What is the reaction mechanism of alkene and bromine?
- polarisation of bromine - the Br molecule is non-polar and it is polaried by the pi bond in a nearby alkene
the electron pair in the pi bond is attracted to the Br+ causing the double bond to break - formation of the intermediate - the pi bond in the alkene breaks and the bromine acts as an electrophile accepting the electrons, the br-br bond breaks by heterolytic fission, a carbocation and bromide ion are formed
- carbocation rapidly reacts with the bromide ion forming the product
What is an electrophile?
an electron pair acceptor
What is a carbocation?
an ion with a positively charged carbon atom
What is the hydration reaction of a alkene?
adding steaming H2O (g)
heat and H3PO4 catalyst used
produces alkane with hydroxyl group - OH
What is Bromination of an alkene?
adding liquid or aqueous bromine breaks the double bond and forms an alkane with two bromines attached
colour change - orange –> colourless if double bond present
What happens when you add a hydride to an alkene eg HCl?
the chlorine atom and the H atom breaks the double bond and forms an alkane
What is the dehydration reaction of an alcohol to form an alkene?
alkane with hydroxyl group heated with H2SO4 catalyst forms an alkene
What is hydrogenation of an alkene?
add H2 heat and nickle catalyst to form a alkane
What is the advantage of combusting polymers?
they are good fuels and burning them releases a lot of energy
also reduces landfill sites
What is the issue with combusting polymers?
produces toxic waste products
How are polymers used in the feedstock for the production of new plastics and other organic chemicals?
polymers can be broken down and the products reacted further to produce a range of other useful organic compounds
What is the advantage of biodegradable and photodegradable polymers?
they break down in the envrionment on a relatively short timescale also reduces landfill sites
Why do flames look different for comustion of alkanes and alkenes?
alkenes have more carbon to hydrogen so more soot is produced
What are all the reactions of alkenes?
hydration- add steam to alkene - alcohol formed
hydrogenation - H2 and Ni = alkane formed
hydrogen halide - makes haloalkane
halogen - makes haloalkane
What reaction forms an alkene?
dehydration of an alcohol with heat and acid catalyst
What test is used to identify C=C (unsaturation)?
alkanes reacted with bromine
bromine water and alkene forms haloalkane
orange solution (bromine water) —> colourless soltuion
- if C=C is present
What determines the carbocation stability?
the electron-donating ability of akyl groups = each alkyl group pushes and donates electrons towards the + charge of the carbocation
more alkyl groups = more the + charge is spread out so more stable
What is a repeat unit?
the specific arrangement of atoms in the polymer molecules that repeats over and over again
What are the uses of poly(ethene)?
supermarket bags
straws
toys
shampoo bottles
What are biodegradable polymers?
made of starch/cellulose and are broken down by microorganisms
What are photodegradable polymers?
oil-based polymers
Which bond are broken in electrophilic addition?
sigma and pi
What bond is made during electrophilic addition?
sigma
