chapter 3- alkenes

what are alkenes?

• unsaturated hydrocarbons

• general formula is CnH2n

• contain at least 1carbon-carbon double bond

what is the shape around the carbon atom of the double bond?

• trigonal planar

• because there are three regions of electron density around each of the carbon atoms

• the three regions repel each other as far apart as possible, so the bond angle around each carbon atom is 120

• all of the atoms are in the same plane

what bonds does the c=c double covalent bond consist of?

• one sigma bond

• one pi bond

how is the pi bond formed?

• by a sideways overlap of 2 p-orbitals on each carbon atom

• this forms a pi bond above and below the plan of the molecule

• the pi bond is weaker than the sigma bond

how is the sigma bond formed?

• overlap of orbitals directly between the bonding atoms

rotation on sigma and pi bonds?

• there can be a rotation around a sigma bond\

• there is a restriction rotation about a pi bond

characteristics of the pi bond?

• they are exposed and therefore have high electron density

why are pi bonds more vulnerable to attack?

• because of their exposure and high electron density

what attacks the pi bonds does?

• species that ‘like’ electrons called electrophiles

electrophile meaning?

• positive ions, or molecules with a partial positive charge, that are attracted to the c=c bond (region of electron density)

• electron pair acceptor

why are alkenes reactive?

• the c=c bond has a greater electron density and therefore a greater negative charge

• therefore, molecules can react with alkenes by accepting a pair of electrons from the c=c bond

• electrophiles are attracted to the c=c bond

  • most reactions with alkenes involve electrophiles

testing for alkenes?

• add bromine water to the alkene at room temperature

• observation: orange colour of bromine water will decolourise to colourless

• it forms a dibromoalkane

what is more reactive: alkanes or alkenes?

alkenes because of the double bond

why is there stereoisomerism in alkenes?

• because of the presence of double bonds

  • unlike single bonds, double bonds are rigid and you cannot rotate around them

• their single bonds allow free rotation

  • so alkenes display stereoisomerism at the double bonds

• also, each carbon atom of the double bond is attached to 2 different groups

definition of stereoisomers?

• same structural formula

• different arrangement of atoms in space/different spatial arrangement of atoms

definition of E/Z isomerism?

• example of stereoisomerism, in terms of restricted rotation about a double bond and the requirement for 2 different groups to be attached to each carbon atom of the carbon=carbon double bond

definition of cis-trans isomerism?

• special case of E/Z isomerism in which 2 of the substituent groups attached to each carbon atom of the c=c group are the same

what is the reaction of an alkene with bromine/chlorine an example of?

electrophilic addition

what happens in the reaction between ethene (an alkene) and bromine? step 1

1) as the Br2 molecule approaches the alkene, the pi bond electrons repel the electron pair in the Br-Br bond, this then induces a dipole

• the pi bond repels the electron pair, because the pi bond is a region of high electron density- electrons are negative in the alkene, and in the br bond, like charges repel

• this induces a dipole, so the Br2 becomes polar and electrophilic

what happens in the reaction between ethene and bromine step 2?

• the bromine delta + is attracted to the electron rich pi bond

• the 2 electrons in the pi bond come out to make a covalent bond with the positive bromine of the br2

• the br-br bond breaks heterolytically

• one Br attaches to one of the carbon atoms, and a bromide ion is formed with the other

what happens in the reaction between ethene and bromine, step 3?

• the bromide ion contains a lone pair

  • it acts as a nucleophile and is attracted to the positive carbon on the carbocation, forming a new covalent bond

• overall, there is an addition of br2 across the alkene

why does the pi bond give out electrons?

because the pi bond has a high electron density and is unstable

• they are ‘looking’ for a way to get more stable

why does the br-br bond break heterolytically?

because one of the bromines is now covalently bonded to the carbon                                                                

electrophilic addition of water- characteristics?

• only the first stage is an electrophilic addition reaction

• needs a catalyst (phosphoric acid absorbed onto silica)- used to make ethanol

• high temperature of 400C and Pressure 60atm

• concentrated sulfuric acid is used in the lab, hydration reaction

what would happen to the addition of hydrogen bromide if the alkene is unsymmetrical (markownikoff’s rule)?

• it can lead to 2 isomeric products because there are 2 different possibilities where the bromine could attach to the alkene- either side of the double bond

do the 2 products form in equal amounts?

no

• one is called major and one is called minor

what rule do you use to determine whether it is a major or minor product?

• markownikoff’s rule

  • in most cases, the bromine will be added to the carbon with the fewest hydrogens attached to it

example of markownikoff’s rule with the formation of bromobutane from but-1-ene and hydrogen bromide?

• the 2-bromobutane forms much more often (90%) and is called the major product

• the other possibility- 1-bromobutane- is called the minor product and forms much less (10%)

what catalyst is used in the electrophilic addition of an alkene and hydrogen?

(to form an alkane)

nickel catalyst

• also known as a hydrogenation reaction

what catalyst is used in the electrophilic addition of water?

(to form an alcohol)

an acid catalyst, such as phosphoric acid or sulfuric acid catalyst

what happens in addition polymerisation?

• the alkene molecules add to themselves to form long chain molecules

• the double bond in the alkene opens to bond to the next monomer molecule

what is the original alkene molecule called in addition polymerisation?

• monomer

how do chains form?

• when the same basic unit is repeated over and over

what is the polymer of ethene called?

(poly)ethene

why are polyalkenes unreactive?

• because of the strong c-c and c-h bonds

what are the bonds in polymer chains?

• strong covalent bonds

  • between different polymer chains there are weaker intermolecular forces (london forces and permanent dipole forces)

what does increased chain length lead to in polymers?

• increasing density, melting point and hardness

uses of polymers?

• they are unreactive which makes them useful for manufacturing everyday plastic products

• however, due to them being unreactive, it means they are not biodegradable

• therefore, they may go to landfill

how to dispose polymers?

• waste polymers could be used as organic feedstock to produce plastics and other organic compounds

• recycled

• some are combusted to produce energy for other industrial processes

what happens during combustion of polymers?

• they can release toxic gases which must be removed to reduce the impact of the environment

advantage of combustion?

advantage: energy used to produce electricity

disadvantages of combustion?

• formation of hcl

• releases co2/gases that cause global warming

benefits to the environment of development of biodegradable and photodegradable polymers?

• reduced long-term pollution

• alleviating problems from disposal of persistent plastic waste

• less harm to wildlife

• reduced landfill use