Topic 13 alkenes Chemistry Dooley

13.1- 13.5

Define unsaturated. (2)

An organic compound containing at least one C to C double bond or triple bond or an aromatic ring. Therefore not containing the greatest possible number of hydrogen atoms.

Describe alkenes.

Alkenes are unsaturated hydrocarbons that are aliphatic which have the general formula of CnH2n.

Define the two bonds that make the alkene double bond. (2)- May draw diagram- check in notebook.

The alkene double bond is made up of 2 types of bonds:

1. Sigma bonds- which is formed by the overlap of orbitals directly between the bonding atoms.

2. Pi bond- formed by the sideways overlap of adjacent p orbitals above and below the bonding C atoms.

Why can sigma bonds rotate but pi bonds can’t ? (2)

Sigma bonds can rotate as they are formed by head on overlap which result in a symmetrical and cylindrical electron denisty.

But Pi bonds are formed by side by side overalp that are parallel, rotation would destroy the parallel alignment of the p orbitals.

Explain the shape and bond angle around each carbon atoms in the double bond of an alkene? (4)

There are only three bonding regions in each carbon atoms with double bonds.

The shape is trigonal planar,

due to the electron pair repulsion theory, the three regions repel each other as far as possible,

forming bond angles of 120 degrees.

Define steroisomerism.

Compounds with the same structural formula but a different arrangement of the atoms in space.

Define structural isomerism

Molecules with the same molecular formula but a different structural formula.

Explain chain, position and functional structural isomers.

Chain= different carbon chain

Position= different location of a functional group

Functional- different functional group

Describe E-Z stereoisomerism.

It is a system that tells whether the higher priority side is on the same or opposite sides.

This only exist in double bonds because of the fixed rotation.

What is a rule for E/Z isomerism?

Each carbon on the double bond must have different functional groups attached.

What is E isomerism?

When the highest priority is on opposite sides (diagonal)

What is Z isomerism?

When the highest priority is on the same side.

What is Cis- Trans isomerism?

A special case of E/Z isomerism where one of the functional groups on each carbon must be a hydrogen.

What is Cis isomer?

Hydrogen groups are on the same side

What is Trans isomer?

Hydrogen groups are on diagonal side.

Explain the CIP priority rules

Exam the atoms attached directly to the carbon atoms of the double bond and decide which of the two atoms has the highest priority or highest atomic number.

If the atoms attached to the carbons in the double bond are the same, then go along the chain until you find the first difference and find the higher priority/ atomic number.

Explain why alkenes are more reactive than alkanes in terms of their bonds.

Alkene’s double bonds have pi bonds that are

• The electrons are more exposed to electrophiles (that attacks) because it has sideways overlapp above and below

• It has a lower electron density between the nuclei so it breaks with less energy

• Easily open up the double bonds to form 2 new single bonds during an additional reaction

Alkane only have strong,single, stable sigma bonds that are

• less accessible because they have direct overlaps that held tighly between the nuclei- takes more energy to break

Describe what an electrophile addition reaction is.

When an electrophile attacks a C to C double bond and breaks the pi bond to form two new single sigma bonds, joining new atoms to the molecule.

Define and give examples of electrophile.

An electrophile is an electron- pair acceptor and they are usually positively charged ions (cations) like H+ or in polar molecules like H-Br .

Name 4 addition reactions and include the reactants and products.

1. Hydrogenation- Forming alkane from alkene and hydrogen

2. Halogenation- Forming dihaloalkanes from alkene and halogen

3. Forming halogenalkanes from alkene and hydrogen halides

4. Hydration- Forming alcohol from alkene and water

What are the conditions needed for hydrogenation?

Nickel catalyst under 150°C

Why is hydrogenation not an electrophile addition reaction?

Because no electrophile is present to attract the electrons on the double bond.

H2- hydrogen has strong H to H covalent bonds and they are not easily polarized even when near an alkene.

Give an example of hydrogenation in food.

Where unsaturated liquid vegetable oil is turned into saturated solid margarine.

What is the condition needed for halogenation?

Alkene and Halogen reacts in room temperature.

How can halogenation help to identify/ test for unsaturation?

Bromine water turns from an orange- brown solution into a colourless solution when it reacts with unsaturated alkenes.

The colourless solution of saturated dibromoalkane is formed.

Explain how bromine act as an electrophile in halogenation.

Bromine is usually non polar, but when it approaches an alkene, it is polarized (induced) by the pi bonds due to its high electron density- (where it repels the electrons in the bromine bond).

Describe the mechanism of the electrophile addition reactions in halogenation (for bromine)- draw the steps. (5)

1. The double bond attacks the partial positive side of the bromine (where the arrow goes from the double bond to the partial positive)- the double bond breaks

2. One of the new sigma bonds formed between the carbon and the partial positive Br

3. Br to Br bond breaks heterolytically where both electrons goes to the partial negative side of the Br end of molecule. (arrow goes from Br to Br bond to partial negative Br)

4. A bromide ion and a carbocation are formed

5. They react to form a bond and lastly a dibromoalkane is formed.

What condition is needed when forming haloalkanes?

Gaseous hydrogen halides at room temperature

Describe the mechanism for the electrophile addition reactions to form halogenalkanes. (4)

1. Bromine is more electronegative than hydrogen so it becomes the partial negative one while hydrogen is the partial positive one.

2. The double bond is broken when it is attracted to the partial positive hydrogen. (arrow from the double bond to the partial positive hydrogen)

3. A bond forms between the carbon and the hydrogen while the electron pair in the H to Br bond goes to the bromine atom (bond between H-Br bond breaks heterolytically = arrow going from bond to Br)

4. A bromide ion and a carbocation are formed and lastly they react together and form the haloalkane.

What are the conditions needed for hydration to occur?

Alkenes need to react with steam and phosphoric acid/ sulfuric acid catalyst under 300°C and 60 atm.

What other kind of reaction is alkene forming alcohol with steam?

Reverse reaction

Describe the mechanism for the electrophile addition reactions of hydration. (4)

1. The double bond is broken when the Pi electrons attacks the H+ (from the acid) which forms a new C to H bond.

2. Electron pairs from the water attacks the carbocation and reacts to form a bond in between.

3. One of the hydrogen from the water will be removed as H+ and can be regenerated (as oxygen can only form 2 bonds)

4. At last the H+ is released back to the solution while an alcohol is produced as a single addition product.

Define an nucleophile.

An electron- rich specie that can donate pairs of electrons to an electrophile.

Describe the Markownikoff’s rule. (3)

This rule predicts the major product (from 2 different isomers that could be formed) in an addition reaction of asymmetrical alkenes.

When a hydrogen halide reacts with an asymmetrical alkene, the hydrogen atom is more likely to bond to the carbon atom that is attached to more hydrogen atoms.

While the halide ion is more likely to attach to the carbon with more alkyl groups.

Explain the Markownikoff’s rule with carbocation intermediates and stability. (3)

The hydrogen attaches with the carbon with more hydrogen because the carbocation wants to be created on/ the postive charge wants to land on the carbon that is more stable (with more alkyl groups)

Tertiary carbocation are the most stable (3 alkyl groups) and the primary carbocation are the least stable (1 alkyl group).

The stability of the carbocation increases with the number of alkyl groups attached to the carbon.

Explain why the major product is often formed by a more stable carbocation intermediate. (1)

Because a more stable carbocation intermediate requires less activation energy so the product will be formed more often.

Describe and explain how the stability of the carbocation increases with the number of alkyl groups attached to the carbon with inductive effect.(3)

The inductive effect states that:

• Alkyl groups donates electron density to positive charged carbons

• This reduces the intensity of the positive charge

• Spreading this positive charge makes the intermediate more stable

Define polymers.

Large molecules formed form many repeat units of smaller molecules (monomers)

Define addition polymerisation.

The formation of a long chain, saturated polymer where the double bonds in the unsaturated monomers will be broken.

Define substituted alkene.

An organic molecule containing a carbon to carbon double bond where one or more hydrogen atoms are replaced by alkyl groups or other functional group.

What does the monomer and polymer usually look like and where are their number of units placed?

The monomer is usually in double bonds and the number of units is placed infront of the monomer unit

The polymer is usually in single bonds and in a bracket with two bonds sticking out on each sides. The number of units is placed at the bottom corner after the bracket.

Name 5 different polymers and give examples of their different uses

1. Poly(ethene) = supermarket bags, toys, bottles

2. Poly(chloroethene) = Flexible/ rigid pipes, insulating cables

3. Poly(propene) = Toys, windows, guttering

4. Poly(phenylethene) = Packaging, food trays, cups

5. Poly(tetraflouroethene) = Coating non-stick pans, cables insulation

What are the 4 advantages of polymers.

1. Available

2. Cheap to purchase

3. Convenient- lightweigh and flexible

4. Suitable for storing food or chemicals safely

What is an disadvantage of polymers?

The unstable property makes it non biodegradable which impacts the environment because they are hard to dispose.

What are the 5 different ways of how waste plastics are disposed?

1. Landfill

2. Recycling

3. Combustion

4. PVC recycling

5. Feedstock recycling

What are the 2 advantages of using landfill as a way of disposing plastics?

• When the waste plastic is hard to be seperated from other waste

• Or when there are not enough plastic to be extracted to make it economically viable

What is the disadvantage of using landfill as a way of disposing plastics?

Non-sustainable as large area of surface is used

What are the 3 advantages of recyling as a way of disposing plastics?

1. Reduce the use of crude oil and fossil fuels

2. Decrease waste going into landfill

3. Reduce the use of finite resources

What is the disadvantage of recycling as a way of disposing plastics?

Very time consuming as many steps are needed that includes:

sorting into different categories, chopped into flakes, washing, dried, melted, cut into pellets then make new products or cracking into monomers for original feedstock.

What are the advantages of combustion as a way of disposing plastics?

• Can be used when the plastics cannot be recycled

• And can generate electricity by making steam

What is the disadvantage of combustion as a way of disposing plastics?

Emit greenhouse gas- cause global warming

How are PVC being recycled?

Solvents are used to dissolve PVC into precipitateswhich can be reused.

What are the 3 disadvantages of disposing PVC?

1. Recycling PVC is hazardous due to high chlorine content and range additives

2. Putting PVC in landfill is not sustainable

3. PVC releases HCl which is a corrosive gas as well as other pollutants when burnt.

Describe what feedstock recycling is and its advantages.

This is when waste polymers are broken down by chemical and thermal processes into monomers, gases and oils.

• Products can be used as raw materials to make new polymers and other organic chemicals

• Compare to other polymer disposal, they don’t need to be washed or sorted.

What are the 2 disadvantages of feedstock recycling?

• Expensive

• High energy demand

Explain how bioplastic is a better alternative to oil- based products.

Bioplastics are produced from plant starch, cellulose, plant oils and proteins.

• They are renewable and sustainable

• It conserves valuable oil reserves

Describe the two environmentally friendly polymers.

1. Biodegradable polymers can be broken down by microorganism that turned into H2O, CO2 and biological compounds. They are made from plant materials or additives that alter structure of traditional ones. They can be used as bin bags for food waste so they can be compost together.

2. Photodegradable polymers are oil- based polymers that have bonds weakened by absorbing light.