13 Alkenes

pi bond π

the sideways overlap of two p-orbitals, one from each carbon atom of the double bond

stereoisomers

molecules that have the same structural formula but a different arrangement of atoms in space

conditions for a molecule to have E/Z isomerism

• a C=C double bond

• each C in the double bond must be attached to two different groups

conditions for a molecule to have cis/trans isomerism

• C=C double band

• each C in the double bond must be attached to two different groups

• one of the attached groups on each C atom must be a HYDROGEN

E and Z meaning

E = opposite sides

Z = zame zide

cis and trans meaning

trans = across (E)

cis = same side (Z)

skeletal formula for (Z) pent-2-ene

skeletal formula for (E) pent-2-ene

y r alkenes more reactive than alkanes

double bond: pi-bond readily breaks as it’s on the outside of the double bond

reactions of alkenes

1. hydrogenation of alkenes

2. halogenation of alkenes

3. alkenes + hydrogen halides

4. hydration of alkenes

equation for hydrogenation of alkenes

equation for halogenation of alkenes

equation for alkenes + hydrogen halides

equation for hydration of alkenes

conditions for hydrogenation of alkenes

• nickel catalyst

• 423K

what is hydrogenation of alkenes used for

solidification of vegetable oils

• higher mpt as the chains are packed together more, greater SA in contact, greater LF

conditions for halogenation of alkenes

no conditions

describe a test for unsaturation

• add orange bromine water

• will react w unsaturated if present - colour disappears

• w alkane stays orange

what’s another name for a successful test for unsaturation

halogenation of alkenes

equation for alkenes + hydrogen halides

conditions for alkenes + hydrogen halide

GASEOUS hydrogen halide (bubble it through alkene if liquid)

how many products can be formed in alkenes + hydrogen halide

2, the H and X can go to either C atom when the C=C opens up

equation for hydration of alkenes

conditions for hydration of alkenes

• STEAM

• 300C

• Acid catalyst: PHOSPHORIC ACID H₃PO₄

• _{60-70atm???idk}

product of hydration of alkenes

alcohol

what is the shape and bond angle around a C=C

trigonal planar

120

what is the bond angle for cyclopropane and why

60 due to bond strain

what is the mechanism for addition reactions of alkenes and WHY

electrophilic addition

• double bond, region of high electron density because of pi-electrons

• attracts electrophiles

• susceptible to electrophilic attack

electrophile def

electron pair acceptor

how can non-polar molecules act as an electrophile

• the electron rich pi-bond repels electrons in non-polar molecules (like repels like)

• induces a dipole

carbocation / carbonium ion

an ion containing a positively charged carbon atom

stabilities of carbocations

most stable carbocation

tertiary carbocation

how many Rs and Hs does a primary carbocation have

1xR

2xR

how many Rs and Hs does a tertiary carbocation have

3xR

no H

why is a secondary carbocation more stable than a primary carbocation

alkyl inducing effect

what is the major and minor in relation to stability

major: most stable

minor: least stable

polymers

large molecules formed from many thousands of repeat units of monomers

polymerisation

the joining of a very large number of monomer molecules to make a large polymer molecule w a higher molar mass

what happens when alkenes undergo addition polymerisation

the C=C double bond breaks open

what is required for industrial polymerisation

• high temp

• high pressure

• catalysts

why are polymers unreactive

• non polar

• no pi-bond, only sigma bonds

• many LF as v large

how can structures (and therefore properties) of polymers made of the same monomer be changed

altering the conditions in which the polymers are formed

HDPE

• formed w use of catalysts and higher temps

• harder

  • high SA contact w other molecules

  • greater LF

does HDPE have more or less branching than LDPE

HDPE has low degree of short chain branching

LDPE has high degree of short chain and long chain branching

y r plastics useful

• readily available

• cheap

• convenient to throw away

• can store food

pros and cons of recycling plastics

why is disposal of PVCs hazardous

• high chlorine content and range of additives present

• dumping in landfill not sustainable

• released hydrogen chloride HCl (a corrosive gas) and other pollutants like toxic dioxins

what new technology is there to dispose/recycle PVC

• solvents dissolve polymer

• high grade PVC can be recovered by precipitation from solvent

• solvent can be used again

why are waste polymers w a high stored energy value useful

• can be incinerated to produce heat

• generating steam

• drives turbine

• electricity produced

feedstock recycling

the chemical and thermal processes that can reclaim monomers, gases or oils from waste polymers

• products resemble those produced in crude oil refineries

• i.e. raw materials for production of new polymers

• able to handle unsorted and unwasted polymers

what are bioplastics produced from

plant starch, cellulose, plant oils and proteins

benefits of bioplastics

• renewable and sustainable alternatives to oil based products

• protects environment and conserves oil reserves

what are biodegradable polymers often made from

starch or cellulose

how are biodegradable polymers broken down

microorganisms

what is the chemistryey name for expanded polystyrene

poly(phenylethene)

what are expanded polystyrene plates/cups/food trays being replaced by

sugar cane fibre plates/cups/food trays

what are photodegradable polymers made of

oil based polymers

how do photodegradable polymers degrade

they contain bonds weakened by absorbing light to start the degradation

what is LDPE used for

plastic bags/films (plastic w little strength)

properties of LDPE

flexible, moisture-resistant material w elasticity

what is HDPE used for

children’s toys, water pipes (plastic w strength)

properties of HDPE

sturdy, durable, good chemical resistance

what is polypropene used for

food containers, kitchen equipment

what are PVCs used for

water pipes, waterproof clothing

films and sheeting, insulation and cable sheathing

the other name for PVC

polychloroethene

structure for polystyrene

other name for polystyrene

polyphenylethene

what is polystrene used for

packaging material, food trays and cups

properties of polystyrene

insulating

what does PTFE stand for

poly(tetrafluoroethene)

structure for PTFE

another name for PTFE

teflon

what is teflon used for

coating of non stick pans, permeable membrane for clothing and shoes

properties of PTFE/teflon

slippery