chapter 13- alkenes

What is the general formula for branched/alkenes?

CnH2n

(Cyclic alkenes and alkenes with more than one double bond do not follow this rules)

Define alkene.

homologous series of unsaturated hydrocarbons with at least one c=c double covalent bond comprising of a sigma bond and a pi bond

What is the double bond made up of?

Sigma and pi bonds

Why do double bonds have a high electron density?

Because they have 4 shared electrons within a single region

(Whereas in a single bond they only have 2 shared electrons)

What do double bonds contain?

A sigmas and pi bond

What is the sigma bond?

When two s orbitals overlap and align horizontally to give a single covalent bond

When the two s orbitals overlap horizontally what happens?

It gives the highest possible electron density between the two nuclei creating a single covalent bond

Why do sigma bonds have a high bond enthalpy?

Because the high electron density between the nuclei means there is a strong electrostatic force between the nuclei and the shared pair of electrons

Define bond enthalpy.

The energy required to break 1 mole of covalent bond in gaseous state.

(So the more electrons there are in a single region the higher the bond enthalpy due to electrostatic attraction of shared electrons and nuclei)

Define electrostatic atttraction.

A force that pulls particles with opposite electrical charges together.

What is the strongest type of covalent bond?

Sigma bonds

What is a pi bond?

The parallel overlap of two p orbitals

Why are pi bond weaker (lower bond enthalpy/easier to break) than sigma bonds?s

Because the electron density is spread out above and below the nuclei, therefore weaker electrostatic attraction and lower bond enthalpy

(Whereas sigma bonds have a direct head- on overlap.)

Why do pi bonds have relatively low bond enthalpy?

Because they have weak electrostatic attraction between the nuclei and the shared pair of electrons because they are spread out above and below the nuclei

What makes the alkene so reactive?

The pi bond has a low bond enthalpy- so easier to break- and spread out above and below the sigma bond, making them exposed (susceptible to electrophiles- higher chance of reaction)

What is a nucleophile?

An electron pair donor

(So atom/s that are attracted to an electron deficient carbon- because of the C=C bond)

What is a electrophile?

Electron pair acceptor

(So they are deficient in electrons)

What are two examples of electrophiles.

1. Positively charged ion

2. Polar molecules (since the positive dipole is attracted to the place with lots of electrons)

Are nucleophiles attracted to electrophiles?

Yes

Define non polar molecule.

A molecule where there is no overall dipole

Why are nucleophiles not attracted to non polar molecules?

1. Have equal/similar electronegativities- so pull on shared electrons is equal

2. They’re symmetrical-

Because the molecule lacks an electron deficient center (positive charge). Therefore there is no electrostatic attraction to overcome repulsion between the nucleophile and the molecules electron cloud

Why are alkenes more reactive than alkanes?

1. Because the C=C bond contains sigma and pi bonds

2. Higher electron density- 4 shared electrons

3. Pi bonds in alkenes have a low bond enthalpy and more susceptible to electrophiles

Define stereoisomers.

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

What is the difference between E/Z isomerism vs cis/trans isomerism.

E/Z isomerism looks at atomic number (low/high priority)

Whilst cis/trans isomerism looks for identical groups

What is the shape of an alkene?

Trigonal planar

What is the bond angle of an alkene?

120 degrees

Why is the bond angle of an alkene 120 degrees?

1. It has 3 bonding regions (regions of electron density)

2. 0 lone electron pairs

Why can’t atoms rotate around C=C?

Because they are planar

C=C are rigid. Define rigid.

The structure cannot change its shape (without breaking a chemical bond)

Are alkanes rigid

True

How do you know if a molecule is E isomerism?

Same group opposite the double bond

How do you know if a molecule is a Z isomerism?

Same groups on the same side of the double bond

What is the test and result for alkenes

Test- add bromine water to the solution

Result- if alkene is present colour will change from brown-orange to colourless

How are alcohols formed?

By the hydration of alkenes

What does steam + acid catalyst create?

An alcohol

What is the temperature, pressure and catalyst needed for alkenes to react with steam (hydration of water)?

Temp- 300*C

Pressure- 60 atm

Catalyst- phosphoric acid

How can you make hydrating alkenes produce a larger yield?

Recycle/ re use the unreacted alkene gas

What conditions are needed to turn an alkene into an alkane when adding hydrogen to the C=C (addition reaction)?

1. Nickel catalyst

2. 150*C

What happens when you react hydrogen halides with unsymmetrical alkenes?

2 different products are formed

How is the amount/yield of the 2 products determined?

By the stability of the carbocation

What makes the carbocation intermediate more stable?and why do they make the intermediate more stable?

if there are more alkyl groups (hydrocarbons) bonded around it

Why? Because more alkyl groups push electrons towards the positive carbocation stabilising it.

Explain the Markownikoff’s rule.

the electrophilic addition of a hydrogen halide to an unsymmetrical alkene, the hydrogen atom adds to the carbon atom of the double bond that has the greater number of hydrogen atoms (and the smaller number of carbon atoms) attached.

When do you use landfill to dispose of plastics?

1. When plastics are too difficult to recycle

2. To difficult to separate from other materials

3. Not enough plastic to extract to make it economically viable

What are the disadvantages of landfill.

1. Not very sustainable (large amounts of land us neeeded)

2. Increasingly expensive to use land for waste disposal (land tax)

   therefore need to reduce reliance on landfill (incentivising recycling schemes)

Why are most plastics non renewable?

Because they are made from crude oil

Describe an advantage of recycling.

Reducing reliance on crude oil for making plastics

What are ways to make disposal of polymers more sustainable?

1. Melt and re-mould into new objects or recycle it

2. Crack (brake up polymer chain)waste polymers into a monomer to be used as an organic feedstock to make new plastics/other chemicals

3. Incinerating (burning) waste plastics which couldn’t be recycle- energy from burning can be used to generate electricity

4. Develop biodegradable/photodegradable polymers

what is the downside of incinerating/ burning plastics?

1. Toxic fumes are released- particularly chlorine based plastics (e.g. PVC)which produce harmful HCL gas when burned.

2. Needs to carefully monitored and controlled

What can be done to neutralise the acidic gases produced (HCL) when burning plastics?

Use a flue gas scrubber to neutralise the acidic gases.

How do flue gas scrubbers work?

They fire a base at the flue gases (to neutralise gas)

What are biodegradable polymers?

Polymers that decompose naturally under conditions by organisms

How are biodegradable polymers made?

1. Oil fractions

2. Renewable source- starch

Are biodegradable polymers more expensive non biodegradable equivalents?

Yes

What are the conditions needed for biodegradable plastics to degrade?

Good supply of oxygen and moisture

what are some uses of biodegradable plastics?

Used In frost protective sheets for plants-

which break down over time due to micro organisms so don’t need to dispose of the old sheeting

Explain what a photodegradable polymers is.

They degrade when exposed to sunlight