Alkenes Flashcards by stan twice

Structural isomerism definition

Molecules with same molecular formula but different structural formulas

How well did you know this?

Not at all

Perfectly

Structure of alkanes

Saturated hydrocarbons
General formula C2H2n+2
Covalent bonds around each carbon are a tetrahedral structure
Bond angle (109.5)
Contains sigma bonds
Not fixed in place (single bond can rotate)

How well did you know this?

Not at all

Perfectly

Features of alkane

Carbon atoms can rotate (due to sigma bond)
Unreactive bc strong covalent bond
High mp/bp
Insoluble in water

How well did you know this?

Not at all

Perfectly

What increases strength of an alkanes bond

As the chain gets longer
More vdw
So stronger bonds between molecules
Also more SA
So larger place for vdw forces to happen

How well did you know this?

Not at all

Perfectly

Why are branched alkanes weaker than unbranched ones

Branches prevent molecules getting close to each other
So vdw forces are reduced as they act over longer distances

How well did you know this?

Not at all

Perfectly

Uses of alkanes

Non renewable fossil fuel
Starts process for making pharma
Factories

How well did you know this?

Not at all

Perfectly

Crude oil

Fossil fuel containing alkanes
Formed underground from remains of dead plants and animals
Heat and pressure compresses the remains to get crude oil

How well did you know this?

Not at all

Perfectly

Separation of alkanes: fractional distillation

Heat and vaporise crude oil
Vapour enters fractionating column
Has temperature gradient (hot at bottom, cool at the top)
Longer chain alkanes have stronger vdw so they remain at the bottom of column
Shorter chain alkanes have weaker vdw so they float to the top
This can then the removed by condensing and being removed from column

How well did you know this?

Not at all

Perfectly

Why is shorter chain hydrocarbons more useful than longer chain hydrocarbons

Can be combusted easily for production of car fuel (petrol)

How well did you know this?

Not at all

Perfectly

What is cracking

Breaking down larger hydrocarbon chains (alkanes) into shorter alkanes and alkenes

How well did you know this?

Not at all

Perfectly

Thermal cracking

High temp (900•c)
High pressure (70 atm)
Splits a long alkane into alkanes and alkenes or radicals

How well did you know this?

Not at all

Perfectly

Catalytic cracking

Low temp (450•c)
Low pressure (2 atm)
Uses zeolite catalyst
Often produces branched alkanes

How well did you know this?

Not at all

Perfectly

Why are alkanes unreactive

Non polar (c-h bond has similar electronegativity)
Contains strong covalent bond bonds

How well did you know this?

Not at all

Perfectly

Free radicals

Unpaired electron in covalent molecule (shown as dot)
Highly reactive

How well did you know this?

Not at all

Perfectly

How well did you know this?

Not at all

Perfectly

Describe the process of free radical substitution (all three steps)

Initiation step:
Break halogen into two radicals using UV radiation
(E.g. Cl2 -> Cl• + Cl•)

Propagation stage:
Any two chain reactions to make desired radicals

Termination:
Form a molecule with no unpaired electron

Nucleophilic substitution of halogenoalkane using :OH- or :CN-

Curly arrow from middle of bond to the halogen (e.g c-Cl)
Halogen breaks off to form negative ion
Carbon is now electron deficient
So electron donor nucleophile donates lone pair to C
(Draw curly arrow from middle of lone pair to c)

What is a nucleophile and give examples

Has lone pair of electrons that can be donated to electron deficient C

:OH- (Hydroxide)
:CN- (Cyanide)
:NH3 (Ammonia)

Conditions for a nucleophilic substitution

Presence must be in aqueous ethanol solvent
Heat (100•C) reaction under reflux

With ammonia, ammonia must be in excess in a sealed tube

Produces alcohol

Expeiment: rate of reaction of halogen alkanes

Add 1cm3 of ethanol solvent in test tube for each alkane
Add 0.1cm3 of diff halogenoalkane in each
Place all test tubes into water bath (60•C)
In separate test tube add aq silver nitrate and put in water bath too
Wait for 10mins (all solution same temp)
Add 1cm3 of silver nitrate to each test tube
Time reaction till ppt forms

Why does silver nitrate react with halogens

Ag+ ions react to the X- (halogen ion)
To form insoluble ppt

Ppt of halogen colours

AgCl ppt is white ppt
AgBr ppt is cream ppt
AgI ppt is yellow ppt

Nucleophilic substitution using :NH3

First u react :NH3 to halogenoalkane
1. Draw arrow from middle of halogen bond to the halogen to break it off into - ion
2. Curly arrow from lone pair on :NH3 to C+
Now you add another :NH3 to remove a H
3. Curly arrow from lone pair to H on connect :NH3 on alkane
4. Curly arrow from middle of NH bond to Nitrogen

Elimination reaction

React halogen with either KOH or NaOH dissolved in ethanol
1. Curly arrow from :HO- to a H in alkane
2. Curly arrow from middle of CH bond to the middle of a CC bond
3. Curly arrow from middle of halogen carbon bond to halogen
This produces an alkene and water and Kx or Nax

Conditions for elimination reaction

No water present Uses ethanolic solvent Really hot (1000•C)

Break down of ozone reactions

Desired reactions 1. (In presence of uv) O2 -> O• + O• 2. O• + O2 -> O3 (protective ozone) Consequence of CFC’s 3. uv exposure to cfc causes a Cl• to break off (highly reactive radical) 4. Cl• + O3 -> ClO• + O2 5. ClO• + O3 -> Cl• + 2O2 The formation of cl radical creates a chain reaction leading to destruction of ozone layer

Alkene structure

General formula CnH2n Distance between Each CH bond is 120• as it forms trigonal planar shape P orbitals overlap in alkene to form Pi bond which means alkenes can’t rotate

Why are alkenes more reactive than alkanes

Alkenes have pi bonds (regions of HED) With moving free electrons that can react to other substances

Electrophilic addition reaction steps

1. Curly arrow from middle of double bond in alkene to a slight positive other molecule 2. Curly arrow from middle of external molecule bond to slightly negative atom 3. With the left over negative reactant that has a lone pair, draw curly arrow from lone pair to positive carbon atom Product formed is a halogenoalkane

Test for alkene

Decolourises orange bromine water

Structural Isomers

Molecule w same molecular formula But different structural formulas

Functional group isomers

Have same molecular formula But different functional groups influencing the way they react

Chain isomers

Different arrangements in hydrocarbon chain

Position isomers

Have different positions of functional groups influencing

Steryeoisomers

Molecules with same structural formula but different spatial arrangements of atoms

Cis isomers

Have both functional groups on the same side

Trans isomers

Have functional groups on opposite sides of molecule

E isomers

The highest Mr atoms are on opposite ends of the molecule (enemies)

Z isomers

The highest Mr atoms are on the same side of the molecule (zame side)