Module 4 - Alkenes Flashcards
what are alkenes
-unsaturated hydrocarbons
-CnH2n
describe how sigma bonds are formed in alkenes
-two sp2 orbitals (one from each carbon) overlap to form a single C-C bond called a sigma σ bond
describe how pi bonds are formed in alkenes
-rotation can occur around a sigma bond
-the π bond is formed by sideways overlap of two p orbitals on each carbon atom forming a π-bond above and below the plane of molecule
describe pi bonds
-π bonds are exposed and have high electron density
-they are therefore vulnerable to attack by electrophiles
-the π bond is weaker than the σ bond
describe the structure of alkenes
-planar around the C=C
-bond angle 120 degrees
what are stereoisomers
-they have the same structural formulae but have a different spatial arrangement of atoms
when does E-Z stereoisomerism arise
- there is restricted rotation around the C=C double bond
- there are two different groups/atoms attached both ends of the double bond
define a priority group
-the atom with the bigger atomic number is classed as the priority atom
describe Z isomers
-the priority atom is on the same side of the double bond
describe E isomers
-the priority atom is on the opposite side of the double bond
describe the CIP priority rules
-the atom of higher atomic number is given priority
-f the atoms are the same, consider the atoms at distance 2 from the double bond
describe cis-trans isomerism
-a special case of EZ isomerism in which two of the substituent groups are the same
define an addition reaction
-a reaction where two molecules react together to produce one
describe the conditions needed to convert an alkene to an alkane
-hydrogen reagent
-nickel catalyst
describe the conditions needed to convert an alkene to a dihaloalkane
-bromine reagent, electrophile reagent (Brδ+)
-room temperature
what is the type of bond fission when alkenes react with a halogen
-heterolytic
explain how alkenes react with a halogen (eg bromine)
-as the Br2 molecule approaches the alkene, the pi bond electrons repel the electron pair in the Br-Br bond
-this induces a dipole
-Br2 becomes polar and electrophilic (Brδ+)
-the intermediate formed (carbocation)
describe the conditions needed for an alkene to be converted to a haloalkane
-HCl or HBr reagent, electrophile, HS+
-room temperature
explain how alkenes react with HCl/HBr
-HBr is a polar molecule because Br is more
electronegative than H
-he H δ +is attracted to the electron-rich pi bond
-this reaction can lead to two products when the alkene is unsymmetrical
what is markownikoff’s rule
-in most cases, bromine will be added to the carbon with the fewest hydrogens attached to it
define an electrophile
-an electron pair acceptor
explain what makes a carbocation intermediate more stable
-the more stable carbocation is stabilised because the methyl groups on either (or one) side of the positive carbon are electron releasing and reduce the charge on the ion
how is the major product formed in electrophilic addition
-via the more stable carbocation intermediate
what is the order of stability for carbocations
-tertiary > secondary > primary
describe the conditions needed for alkenes to form alcohols
-steam reagent
-temperature 300-600 degrees
-70 atm pressure
-catalyst of concentrated H3PO4
describe how alkenes form alcohols
-they are reacted with steam in the presence of an acid catalyst
-high pressure means it is done industrially rather than in the lab
advantages of the production of alcohols from alkenes
-high atom economy as no waste products
-separation of products is easier and cheaper to carry out
define a hydration reaction
- a reaction where water is added to a molecule
what is addition polymerisation
-addition polymers being formed from alkenes
reactivity of poly(alkenes)
-like alkanes
-unreactive
-due to strong C-C and C-H bonds
how to draw the polymer repeating unit for any alkene
-draw out the monomer with groups of atoms arranged around the double bond
-then identify the repeating unit
describe the industrial importance of alkenes
-forming polymers from ethene based monomers
-manufacture of margarine by catalytic hydrogenation of unsaturated vegetable oils
what does hydrogenation of liquid vegetable oils do
-converts the double bonds to single bonds
-increasing the melting point of the oil so it is harder and more solid
-forming margarine
how are waste polymers processed
-separation and recycling
-feedstock for cracking
-combustion for energy production
describe how waste polymers are used for separation and recycling
-the waste is sorted into each different type of polymer
-then each type can be recycled by melting and remoulding
impact of waste polymers being used as feedstock for cracking
-allowing for the new
production of plastics and other chemicals
describe how waste polymers can be used in combustion
-can be incinerated and heat is released to generate electricity
-combustion of halogenated plastics can lead to the formation of toxic, acidic waste products such as HCl
describe how chemists can minimise the environmental damage of combustion forming toxic products
-by removing the HCl fumes formed from the combustion process
what have chemists been developing to deal with waste polymers
-biodegradable polymers
-compostable polymers
-soluble polymers
-photodegradable polymers
which polymers are biodegradable
-polymers formed from isoprene (2-methyl-1,3- butadiene), maize and starch
