CL 13- alkenes

Question 1

unsaturated

definition

Answer 1

containing one or more multiple C=C bonds

Question 2

describe a C=C bond

types of bonds etc.

Answer 2

• consists of an overlap of electrons in the 2s subshell to form a sigma bond (3 electrons used)
• remaning electron contained within a p-orbital, which sideways overlap end-to-end to form a pi bond

Question 3

rotation of C=C bonds

Answer 3

no free rotation of the groups about the C atom, as the pi bond extends above and below the plane of the C atoms

Question 4

shape around C atom in alkene

Answer 4

• trigonal planar
• 3 regions containing electrons around each of the C atoms, which repel each other
• bond angle is 120°

Question 5

E/Z isomerism

explain concept

Answer 5

occurs in compounds with a C=C bond, where different groups are attached to each C atom of the double bond

Question 6

cis-trans isomerism

explain

Answer 6

• same conditions of E/Z isomerism, but one of the attached groups on each C atom of the double bond must be the same
• usually the cis isomer is the Z isomer, and the trans isomer is the E isomer

Question 7

how to identify E and Z isomers

Cahn-Ingold-Prelog (CIP)

Answer 7

• assign priority to atoms attached to each C in C=C
• the higher the atomic number, the higher the priority
• if the priority groups are on the same side of the double bond, it is a Z isomer
• if the priority groups are diagonal, it is an E isomer

Question 7

why are alkenes reactive?

Answer 7

• pi bond weaker than sigma bond and easily broken
• unsaturated, so molecules can add across the double bond
• double bond has high electron density, so attractive to electrophiles

Question 7

testing for alkenes

test, result, equation

Answer 7

• bromine water
• from orange turns colourless
• C₂H₄ + Br₂ –> C₂H₄Br₂

Question 7

electrophile

definition

Answer 7

electron pair acceptor

Question 8

ethene + hydrochloric acid
mechanism for electrophilic addition for alkenes

draw and check with written answer

Answer 8

• dipoles on H-Cl (H + and Cl-)
• curly arrow from middle of double bond to H
• curly arrow from middle of H-Cl to Cl
• H connected to C
• C with positive charge, Cl with minus charge, curly arrow from lone pair of Cl to C+
• product of chloroethane

Question 8

use Markownikoff’s rule to predict major and minor

Answer 8

• major product formed from more stable carbocation intermediate
• tertiary most stable, then secondary then primary

Question 8

benefits of ‘combustion for energy production’ of waste polymers

Answer 8

• waste polymers have a high stored energy value as they are derived from petroleum or natural gas
• incinerated to produce electricity for heating homes
• prevents them going to landfill

Question 9

benefits of ‘using as an organic feedstock for the production of plastics and other organic chemicals’ for waste polymers

Answer 9

• feedstock recycling is reclaiming monomers, gases or oil from waste polymers
• used as raw materials to make new polymers
• not gone to landfill, and instead used for another resource

Question 10

benefits of ‘removal of toxic waste products formed during disposal by combustion of halogenated plastics (e.g. PVC)’ from waste polymers

Answer 10

• when burned, PVC releases hydrogen chloride, an acidic gas and dioxins, toxic gases
• as PVC being dumped in landfill is not sustainable
• the PVC can be ground up or dissolved in a solvent and used to make new PVC products

Question 11

benefits to the environment of biodegradable polymers

Answer 11

• broken down into water, CO₂ and biological compounds by microorganisms
• degrade and leave no visible or toxic residues
• usually made from starch or cellulose

Question 12

benefits to the environment photodegradable polymers

Answer 12

• contains bonds that are weakened by absorbing light to start the degradation process