Topic 6- alkenes, alkanes

Homologous series

A family of compounds with the same functional group, which differ in formula by CH2 form the next member

Functional group

An atom/ group of atoms in a molecule that is responsible for its chemical reactions

Structural formula of methyl

CH3

Structural formula of ethyl

C2H5

2 types of structural isomers

Chain isomerism (different length C chain).

Position isomers (functional group is in different positions)

What’s sterioisomerism

its the different arrangement of atoms in space.

What’s E and Z in sterioisomerism

E is Cis, Z is trans

What’s the rule for it to be a sterioisomer

Different groups must be in opposite sides of the double bond.

It can only happen in alkenes due to restricted rotation around the double bond

How do you assign E Z notation if all the groups are different

On the left side, see which has the higher Ar. Do the same omen the left side. Use those two to establish E or Z

How do you assign E Z notation in this situation

What does alkanes being branched have to do with the boiling point

Lowers the boiling point as branches prevent alkane molecules from getting close together, weakening London forces (as they’re stronger the closer together)

Are alkanes polar

Not polar because C and H have similar electronegativities

Type of bonding and forces in alkanes. And reactivity

Covalent bonds called sigma bonds, only London forces. Unreactive cus of strong covalent

Why does strength of London forces increase as chain length increases

Longer chains have more electrons, and longer chain has a greater surface area so more places where London forces can form

What’s thermal cracking and why is it better that catalytic cracking

Heat + steam. Needs high pressure too

Better cus it makes a higher % of alkenes in the product.

What is catalytic cracking

Heat + catalyst

Which hydrocarbons make fuels and feedstock

Fuel = alkane.

Feedstock = alkene

What is reforming and how is it done

Changed straight chain into branched and cyclic for smoother burning in engines.

Done by heating with a platinum catalyst

Complete combustion of propane equations

C3H8(g) + 5O2(g) → 4H2O(l) + 3CO2(g)

2 incomplete combustion of propane equations

C3H8 + 4O2 → 2CO(g) + 4H2O(g) + CO2(g),

C3H8 + 4O2 → C(s) + 2CO2 + 4H2O

Problems with things released from incomplete combustion of alkanes

C - black smoke or soot.

CO- carbon monoxide interferes with uptake of O2 by red blood cells.

HC- unturned hydrocarbons appear as smoke and cause lung irritation and global dimming

All reactions in the oxidisation of sulfur

S(s) + O2(g) → SO2 (g).

2SO2(g) + O2 → 2SO3(g).

SO2 + H2O → H2SO3 (sulphurous acid).

SO3 + H2O → H2SO4 (sulphuric acid)

What is an acidic oxide

And oxide that has the properties of an acid e.g. SO2 and SO3

All reactions in the oxidation of nitrogen

N2(g) + O2(g) → 2NO(g).

2NO(g) + O2(g) → 2NO2(g).

2NO2 + H2O → HNO2 + HNO3

three catalysts in catalytic converters and what three pollutants they remove

Platinum, rhodium, and palladium = catalysts.

HC, CO, NOx = pollutants.

They don’t work properly when cold so pollutants are released in the initial few minutes of a car starting

3 catalytic converter reactions

2CO + O2 → 2CO2,

HC + O2 → CO2 + H2O,

2NO + CO → N2 + 2CO2

What’s biodiesel

Made form vegetable oils mixed with ordinary diesel

What’s bioethanol

Made by fermentation of sugars by yeast. Them distillation to concentrate the ethanol.

Now bacteria is used for fermentation as it gives a higher % of ethanol

Factors to consider when choosing biofuels

Land use, yield, manufacture/ transport, carbon neutrality

What’s a free radical

Any species with an unpaired electron

What’s homolytic fission

Both species formed from the breakup are the same (Cl• and Cl•)

Things needed for the chlorination of methane

at room temp = no traction.

With heat = chloromethane.

At room temp with UV light = chloromethane (photochemical chlorination of methane)

describe the mechanism for the reaction between butane and bromine that forms the products given in the equation bellow:

C4H10 + Br2 → C4H9Br + HBr

Initiation:

Br2 → 2Br•.

Propagation:

C4H10 + Br• → HBr + •C4H9.

Br2 + •C4H9Br + Br•.

Termination:

•C4H9 + Br• → C4H9Br.

•C4H9 + •C4H9 → C8H18.

Br• + Br• → Br2

Why do pi bonds make alkenes more reactive

Bond enthalpy is more in sigma than pi due to pi bonds being from sideways overlap, which is easier to break than the direct overlap in sigma.

There’s a pair of electrons in the sigma bind and a pair in pi. Therefore, the double bond is a region of high electron density, making them highly reactive molecules.

The electrons in the pi blonds are held more weakly and are more unusable to attach and the bond being broken. Pi bonds are weaker than sigma doe to where the shared electrons are found.

Why do addition reactions occurs to alkenes

The pi bond is open to attack and the addition of other species

What are the 5 additions reactions of alkenes

Hydrogenation of alkenes,

Halogenation,

Hydration,

Addition of hydrogen halides,

Oxidation to diols

conditions needed for the hydrogenation of alkenes

150°, nickel catalyst

How is hydrogenation used to make margarine

Hydrogenating vegetable oil.

This prowess increases the melting point making it more solid.

Unsaturated to saturated by breaking double bonds to add hydrogen.

Conditions needed for the halogenation of alkenes

There aren’t any

Conditions needed for the hydration of alkenes

phosphoric acid (H3PO4),

300°c,

60atm

Conditions needed for the addition of hydrogen halides to alkenes

There aren’t any

Conditions needed for the oxidation of alkenes to diols

Potassium manganate(VII) (oxidising agent).

Dilute sulphuric acid (for acidic conditions)

What’s a diol

Contains 2 hydroxyl (OH) groups

What happens when you oxidise an alkene to diols

Potassium manganate (VII) changes from purple to colourless

What’s heterolytic fission

When a covalent bond breaks with both electrons going towards one atom

What’s an Electrophile

electron deficient species (positive ions/ molecules) and can accept an electron pair from electron rich species.

E.g. carbocations and carbonyl compounds

What’s a nucleophile

electron rich species that donates electron pairs to electron deficient species.

e.g. include carbanions, water , ammonia, cyanide ion etc

What’s markownikoff’s rule

When a hydrogen halide reacts with an asymmetric alkene, the H atom of the hydrogen halide is more likely to bond to the C which is attached to the greater number of H atoms

Which is the most stable of 3° 2° 1° carbocations

Most 3° > 2° > 1° least stable

Why does the more stable carbocation give the major product

The positive charge of carbocation can be spread over more atoms, making it more stable, so can exist for a longer amount of time, because alkyl groups are electron releasing

Conditions needed for the addition reactions of alkenes

High temperature and pressure

Polymer of ethane

Poly(ethene)

Why do addition polymers exist in the environment for a long time

Difficult to break down cus they’re unreactive cus their bonds are non polar and relatively strong

ways to fix polymer waste

Recycle, reuse, incineration for energy, chemical feedstock (breaking into H and CO