Alkenes- chemistry

two types of bonds when orbitals overlap in cov bond:

• Sigma bonds (σ)

• Pi bonds (π)

formation of pi-bonds (π)

Side- ways overlap of two p-orbitals

∴ electron density is above and below the molecule

formation of sigma bonds (σ-bond)

end-on overlap of orbitals

carbon- carbon bond in alkenes

carbon atoms use only three of their electron pairs to form a σ bond, each carbon atom will have a p orbital which contains one spare electron

What is a double bond?

combination of a σ and π bond

What is a triple bond?

combination of one σ and two π bonds

Why does bond strength change from single < double < triple bond?

increased electron density around the carbon-carbon bond, making the bond stronger and more difficult to break

What is bonding of carbon of an alkene?

double bond has three σ bonding pairs of electrons

• Two pairs forming σ bonds with other atoms

• One pair forming σ bonds with the other carbon atom of the double bond

The three bonding pair of electrons are in the plane of the molecule and ___ each other

repel

What is the name + bond angle of alkenes?

planar arrangement with bond angles of 120^o 

fourth π bonding pair forms the double bond in combination with the carbon-carbon σ bond 

Ethene’s bonding

• Carbons use 3 of 4 electrons to form σ bonds ( 2 σ with H and 1 σ with other carbon)

• fourth electron from each carbon atom occupies a p orbital which overlaps sideways with another p orbital on the other carbon atom to form a π bond

• means that the C-C is a double bond: one σ and one π bond

What is stereoisomerism?

Compounds with same structural formula but different arrangement in space

Types of steroismicsm:

• Geometric isomerism

• Optical isomerism

What is geometric isomerism:

Can occur with compounds that contain a double bond because there is restricted rotation around the double bond

named depending upon which side of double bond branches lie

cis meaning

latin for on this side

trans meaning

latin for across

trans-but-2-ene

cis-but-2-ene

Difference between cis/ trans and E/Z

• Cis / trans isomerism occurs where two of the atoms or groups of atoms attached to each carbon atom of the C=C bond are the same

• E / Z isomerism is an example of stereoisomerism where different atoms or groups of atoms are attached to each carbon atom of the C=C bond 

The cis / trans naming system can still be used with three atoms / groups of atoms but only if:

• Two of the three atoms or groups of atoms are the same

• These two atoms or groups of atoms are on opposite sides of the double bond

we look at the atomic number of the first atom attached to the carbon in question ∴

The higher the atomic number; the higher the priority

What is E and Z?

Z - highest priorities are opposite each other ———

E- highest priorities are at angles to each other ╲

how to write E and Z notation

e.g.

E-2-bromo-1-propen-1-ol

Z-2-bromo-1-propen-1-ol

if its H2 and H3 what has higher priority?

neither

look at next atom

What in hydrogenation?

• reaction between an alkene and hydrogen is known as hydrogenation or reduction

• As well as a nickel catalyst, this requires a temperature of 200 °C and a pressure of 1000 kPa

Application of hydrogenation

• production of margarine from vegetable oils

• Vegetable oils are unsaturated and may be hydrogenated to make margarine, which has a higher melting point due to stronger London Dispersion Forces

• By controlling the conditions it is possible to restrict how many of the C=C bonds are broken and produce partially hydrogenated vegetable oils which have which have the desired properties and textures for margarine manufacture

What is halogenation?

reaction between alkenes and halogens is known as halogenation

reaction occurs readily at room temperature and is the basis for the test for unsaturation in molecules

What is halogenation an example of

electrophilic addition where an electrophile ('electron seeker') joins onto to a double bond

What does halogenation form

dihaloalkane

What can halogens be used to test for?

If a molecule is unsaturated

What halogen is used to test for unsaturated + how is it done?

• Br₂ is an orange or yellow solution, called bromine water

• The unknown compound is shaken with the bromine water

• If the compound is unsaturated, an addition reaction will take place and the coloured solution will decolourise e.g. 1,2-dibromoethane

What is hydrohalogenation?

Reaction of alkenes readily with hydrogen halides such as HCl and HBr to produce halogenoalkanes

What is hydrohalogenation an example of?

electrophilic addition reaction that occurs quickly at room temperature

Hydrohalogenation reactions in alkenes

Order of fastest to least fast of hydrogen halides and why?

FAST- HI > HBr > HCl- SLOW

due to the increasing bond strength of the hydrogen-halogen bond, so the weakest bond reacts most easily

What is hydration?

When alkene is converted to alcohol

e.g. of hydration

• Treated with steam at 300 ^oC

• Pressure of 60 atmospheres

• Sulfuric acid (H₂SO₄) / phosphoric acid (H₃PO₄) catalyst

• —> water is added across the double bond

• The reaction processes via an intermediate in which H⁺ and HSO₄^-  ions are added across the double bond

• The intermediate is quickly hydrolysed by water, reforming the sulfuric acid

Hydration in alkenes equations

Use of hydration

• important industrial reaction for producing large quantities of ethanol, a widely used solvent and fuel

• The process is much faster and higher yielding that producing ethanol by fermentation

If ethene monomers react what is the name of the polymer formed

Polythene

what do curly arrows represent?

movement of pair of electrons

what is an electrophile?

Atom which is attracted to an electron rich atom, where it accepts a pair of electrons to form new cov bond

electrophilic addition definition

addition of an electrophile to an alkene double bond, C=C, which is an area of high electron density which makes it susceptible to attack by electrophiles. The double bonds breaks forming a single C-C bond and 2 new bonds from each of the two carbon atoms

what is carbocation

an ion that contains a pos charged carbon atoms

A double bond of an alkene is an _____

area of high electron density / high neg charge

electron-deficient species will accept what?

lone pair of electrons

what are electrophiles

means “electron loving”

species either have full pos charge or slight pos charge on one or more their atoms

alkenes undergo addition reactions when approached by ____ and therefore undergo __________

electrophiles

electrophilic addition

1st stage of electrophilic addition

• pos charge on electrophile is attracted to the electron density in the double bond

• as electrophile approaches the double bond, electrons in A-B bond are repelled towards B

• pi bond break and A bonds to carbon, forming carbocation

• Two electrons in A-B bond move to B forming B- ion

2nd stage of electrophilic addition

• The B- ion acts as a nucleophile and attacks the carboncation

• Lone pair of electrons on the B- ion are attracted towards pos charged carbon in the carbocation causing B to bond to it

• both electrons in bond joins B- to the carbocation ion come from B-, the bond is a co-ordinate bond

what is a nucleophile?

species that donates a lone pair of electrons to form a covalent bond with an electron deficient atom

Electrophilic addition of halogens

mechanism for the electrophilic addition of halogens (and hydrogen) is the same as the electrophilic addition of hydrogen halides with one key exception:

• Hydrogen halide molecules have a permanent dipole (as shown above)

• Halogen molecules have a temporary (or induced) dipole caused by the repulsion of the halogens electrons by the high electron density C=C bond

Asymmetric molecules addition

• If e.g. HBr is added to one → 2 possible products can be formed

• 1 product is favoured over other and will be formed in greater amounts

• Amount of each product is decided by intermediate carbocation

e.g. propene + Hydrogen bromide

2-bromopropane

1-bromopropane

Markownikoff’s rule:

when hydrogen halide reacts with an unsymmetrical alkene, the hydrogen of the hydrogen halide attaches to carbon that is attached to most hydrogen atoms and least carbon atoms

The major product will be the one that forms from _____

most stable carbocation (molecule in which a carbon atom has a positive charge and three bonds)

carbocation stability comes from ____

number of electrons releasing groups surrounding it

alkyl groups are what and why?

electron releasing groups as the electrons stabilise the pos charge by reducing it

The mechanism for the electrophilic addition of hydrogen bromide to propene, showing the formation of the major and minor products can be shown as:

stability of the carbocation intermediate is as follows:

tertiary > secondary > primary

Primary carbocation

pos charge is on carbon atom at the end of the chain

secondary carbocation

pos charge is on carbon atom with 2 carbon chains attached to it

Addition polymerisation definition

is the reaction in which many monomers containing at least one C=C double bond form long chains of polymers as the only product

polymer definition

long-chain molecule that is made up of many repeating units

general formula for addition polymerisation

How many products does addition polymerisation form?

1

____ ____ of many polymers makes them ideal for certain use but what does this problem cause

low reactivity

disposal as a lot of polymers are non-biodegradable- environmental problems of waste plastic killing marine animals 

polymer disposal: landfill sites

not ideal and various initiatives are being introduced aiming to reduce this method of waste disposal in general as well as with specific regard to polymers

recycling

• reduces the amount of waste that it going to landfill sites

• reduce the use of finite resources (Lots of polymers are made from the products of cracking crude oil and it's fractions)

• time-consuming as they have to be sorted into the different categories

• After sorting, the polymers are chopped, washed, dried, melted and then cast into pellets ready for use

• Certain polymers can cause problems when recycling due to their chemical composition, e.g. PVC contains a large amount of toxic chlorine which can be released

Combustion

• Some petroleum / natural gas derived polymers are still difficult to recycle

• Since they have a large amount of energy stored within the polymer chains, these polymers can be incinerated

• causes environmental pollution as the carbon within the polymer can be released as carbon dioxide contributing to global warming

• Formation of HCl/products of combustion cause acid rain

• Formation of CO2/gases that cause global warming / greenhouse gases OR Formation of CO

• sustainable?

Feedstock

• where waste polymers are broken down, by chemical and thermal processes, into monomers, gases and oils

• products are then used as the raw materials in the production of new polymers and other organic chemicals

• major benefit of feedstock recycling, compared to other methods of polymer disposal, is that it works with unsorted and unwashed polymers

Bioplastics

• polymers that are made from plant starch / cellulose, plant oils and plant proteins

• provide a renewable and sustainable alternative to the current polymers which are predominantly based on finite resources such as crude oil

Biodegradable polymers

• polymers can be broken down over time by microorganisms

• polyester and polyamide condensation polymers are considered to be biodegradable as they can be broken down using hydrolysis reactions

  • This is a major advantage over the polymers produced using alkene monomers (polyalkenes)

  • When polyesters and polyamides are taken to landfill sites, they can be broken down easily and their products used for other applications

Compostable polymers

• Compostable polymers are commonly plant based

  • Plant starch is being used in the production of biodegradable bin liners

  • Sugar cane fibres are replacing polystyrene in the production of disposable plates and cups

• Compostable polymers degrade naturally leaving no harmful residues

Photodegradable polymers

• Photodegradable polymers contain bonds that are weakened by absorbing light / visible radiation

  • This starts the breakdown of the polymer

• A lot of photodegradable polymers are oil-based 

  • In certain cases, an additive that absorbs light is mixed into the polymer to promote degradation

poly(chloroethene)

PVC / poly(vinyl chloride)

can make polymer flexible or rigid

uses- pipes, films and sheeting, ducts and profiles

poly(propene)

children’s toys, packaging crates, guttering

polystyrene/ poly(phenylethene)

packaging material, food trays + cups

poly(tetraflurethene)

non-stick frying pan

Why is poly-ethanol soluble but poly-1-ene is insoluble in water?

Soluble polymers has an alcohol group which can form hydrogen bonds with water molecules

Possible advantage and disadvantage to environment of disposing of polymers by combustion?

Advantage: Can make heat which is used to make steam to turn turbines to generate electricity

Disadvantage: Using up finite resources of oil to make polymers or toxic gases produced when combusted

Is electrophilic addition homolytic or hetrolytic and why?

Heterolytic, because curly arrows movement of pair of electrons, so bond breaks when both electrons to one of the bonded atom

Using Markownikoff’s rule why is one product major?

Tertiary carbocation is more stable than a secondary carbocation

Biodegradable polymers are often made from plant material and break down through microbial action to produce CO2 and water. People claim these polymers are carbon neutral, why can this can be true and untrue?

True- amount of CO2 released when polymer degrades is = to amount of CO2 taken in by plant photosynthesising

Untrue- Harvesting crop/ manufacturing + transport of crop/polymer likely use fossil fuels therefore more CO2 is emitted than absorbed

Despite having a C=C bond, why cant molecule demonstrate E/Z isomerism

each carbon of C=C double bond does not have different group attached to it

Why cant a molecule be cis or trans?

Each C atom of a C=C bond must have 2 different subsitiution groups

electrophilic addition

Radical substitution produces a mixture of organic products. Suggest two reasons why.

further substitution OR produces different termination products OR More than one termination step✔ substitution at different positions along chain