Chapter 13- alkenes

alkene

a homologous series made up of unsaturated hydrocarbons containing at least one double c=c bond

general formula of alkenes

C_nH_2n

sigma bond

a pair of electrons sideways overlapping s orbitals between the two carbon nuclei

Pi bond

a second pair of electrons are split in overlapping p-orbitals above and below the plane of carbon atoms. This creates an area of high electron density above and below the bond

bond angle of alkenes

three bonding regions, repel equally

trigonal planar shape, 120^o

diaenes/ trienes

alkenes with multiple c=c bonds, every double bond minus 2H from chemical formula

cycloalkenes

alkenes that form a ring, every ring form = minus 2H

stereoisomer

isomers with the same structural formula but with different arrangement in space

two types of isomers in alkenes

1. cis/trans

2. e/z

cis/trans isomerism

when there are only two different groups on each side of the double bond

E/Z isomerism

different groups on each side of double bond

cis isomerism

identical groups on the same side

trans isomerism

identical groups on the opposite sides

how do we decide priority in alkenes?

groups with the highest atomic or molecular weight has higher priority

E isomerism

higher priority are places on opposite sides of the double bond

Z isomerism

higher priority groups on the same side of the double bond

in terms of bonds what happens when alkenes react?

pi bond is broken, sigma bond stays intact

addition reactions

involve the addition of an atom or molecule to each carbon involved in the double bond

four addition reactions in alkenes

• hydrogenation

• halogenation

• haloalkane

• hydration

hydrogenation

alkene + hydrogen→ alkane

conditions: 423K and nickel catalyst

halogenation

alkene + halogen→ dihaloalkane

conditions: room temp and halogen

haloalkane

alkene = hydrogen halide→ haloalkane

conditions: room temp, gaseous

hydration

alkene + steam→ alcohol

conditions: acid catalyst (H₃PO₄), steam

unsymmetrical alkenes

multiple products that are positional isomers

monomer

a small molecule that can join with other monomers to form a polymer

polymer

a large molecule made u of repeating units of monomers bonded together

what makes addition polymers?

addition reactions

what is an environmental advantage of polymers

safe and durable

environmental disadvantage of polymers

do not biodegrade as unreactive, add to landfill problems

options to dispose polymers

recycling, combustion to create energy, organic feedstock

advantages of recycling polymers

preserves fossil fuels, decreases landfill waste

disadvantages of polymers

polymers must be sorted into types to allow for recycling= time consuming

polymers need to be collected

only certain colored plastic can be currently recycled

PVC recycling

can release toxic HCl gas, technology allows for solvents to dissolve the PVC and precipitate allows new PVC to be formed

burning waste polymers to release energy

polymers are deprived from petroleum or natural gases= high energy stores, burnt to heat water in power stations, polymers contain Cl= toxic HCl gas

organic feedstock recycling

reclaim monomers from water polymers that can be used to produce new polymers, it can handle unsorted polymers

what are biodegradable polymers?

produced from plants starch, cellulose, plant oils and proteins

photodegradable polymers

oil based polymers that contain bonds that are weakened by absorbing sunlight this starts the degradation process

mechanism

a step by step sequence of steps by which an overall chemical change occurs

double headed curly arrow represents what

movement of a pair of electrons

electrophile

a lone p[air acceptor- positive charged atom or molecule that is attracted to an electron rich region of a molecule or a lone pair of electrons

carboncation

a carbon based molecule with a positive charge on a carbon atom, it is known as an intermediate as it is very unstable

intermediate

very unstable atom/molecule

mechanism for electrophilic addition

1. electrophile is attracted to the high electron density in the double bond of the alkene

2. the pi bond between both carbon atoms breaks- heterolytic fission

3. the sigma bond in the electrophile breaks-heterolytic fission

2nd step:

1. a carboncation is formed is formed (positive charge) the electrophile now has a lone pair of electrons. Next the species attacks the carboncation using its lone [air to form a covalent bond- overall there is addition

markownikoffs rule

in the electrophilic addition to alkenes the major product is formed via the more stable carboncation

primary structure/ carboncation

the carbon with the positive charge attached to one other carbon atom