Ent

halp

y

Chemical properties of the alkanes

Explain why the old, non-systematic name for the alkanes was the paraffins:

Free radicalsAlmost all alkane reactions involve the formation of free radicals what are they:

Combustion When alkanes are heated in a plentiful supply of air, combustion occurs Alkanes are energetically unstable with respect to water and carbon dioxide They only burn when they are in the gaseous state

Explain what happens when a candle burns:

Sketch and energy profile for the combustion of propane:

Cracking Cracking happens when alkanes are heated in the absence of air

The products of the cracking of long-chain hydrocarbons are shorter chain molecules Ethane is cracked industrially to produce ethene

What are the conditions for catalytic cracking and what are the benefits of using a catalyst?

Write an equation for the cracking of dodecane to form one mole of ethene and an alkane product:

Breaking bonds

Homolytic fission Homolytic fission involves the equal sharing out of electrons in a

bond Each atom receives one electron from the bond when it splits This leaves the atoms with one unpaired electron The atoms are uncharged These ‘free radicals’ are very reactive

Why are free radicals so reactive?

In what type of situation is homolytic fission most likely to occur?

Draw a dot-cross and a curly arrow diagram for homolytic fission of chlorine (NB the curly arrow is wrong in your textbook):

Heterolytic fission Heterolytic fission involves the unequal sharing of the electrons in

a bond Both electrons go to one atom

This results in the formation of two charged particles

In what type of situation is heterolytic fission usually seen?

Draw dot-cross and curly arrow diagrams for heterolytic fission:

Reactions with chlorine Alkanes only react with chlorine when a mixture of the two is exposed to sunlight or

ultraviolet light The light provides the energy required to break the very strong bonds This is an example of a substitution reaction There are three steps to the chain-reaction:

Initiation:The Cl-Cl bond is easier to break than C-H

Propagation:There are several possible propagation steps

Termination:There are also several possible termination steps

Explain the difference in products produced when: methane reacts with a limited supply of chlorine and with a plentiful supply of chlorine:

Chemical and physical properties of alkanes

Chemical properties Alkanes contain atoms of 2 elements: ____________ and

__________

Alkanes are _____________ hydrocarbons

This means that they contain no carbon-carbon ___________ bonds

Alkanes are very _________________ because they contain very __________ bonds

They do burn, and in sufficient oxygen produce _________ and __________ ___________.

Physical properties Alkanes are ________________ in water

In straight-chain alkanes boiling temperature _______________ with chain length; however, this

trend is not followed by branched structural ________________.

ActivityNumber these alkanes in order of increasing boiling point:

Propane Methane Octane Dotriacontane Heptane

1

ActivityLabel the following pairs of structural isomers with highest boiling point (H) and lowest boiling point

(L):

Butane 2-methylpropane

Hexane 2,2-dimethylbutane

2,2,4-trimethylpentane Octane

ActivityBriefly explain the choices you have made in the above activities:

Structural Isomerism

“Isomers are compounds with the same molecular formula but different structural formula”

Structural isomers have the same molecular _____________ but different

molecular ______________. This means that they have the same number of

_________ of the same ___________, but that these atoms are arranged

differently.

Exercise:

Determine displayed, skeletal, structural, molecular and empirical formulae for all structural isomers of the 1st five compounds in the alkane homologous series (methane-pentane), you can use the Molymod kits to help you do this.

The alkenes

Comparison of bonding in ethane and ethene

Ethane The carbon-carbon single bond ( bond) runs through the central axis of the molecule The two ends of the ethane molecule are free to rotate relative to each other

Ethene The ethene molecule is flat Ethene contains a carbon-carbon bond and a carbon-carbon double bond ( bond) The bond lies above and below the central axis The bond prevents rotation around the central axis

The carbon-carbon bond is less than twice as strong as the carbon-carbon bond, why?

What is and electrophile and why do alkanes readily react with them?

Naming alkenes We use the IUPAC naming system to name alkenes The suffix used is –ene We must use a number to indicate the position of the double bond

Complete the table of alkenes:

ethene

but-1-ene but-2-ene

2-methylpent-2-ene

2,4-dimethylhex-2-ene

Alkenes – Chemical reactions

Introduction Alkenes are far more reactive than alkanes They readily react with electrophiles They undergo addition reactions Addition reactions involve heterolytic fission of the

double bond

Electrophiles Species containing and atom with a whole or partial ________________ charge

They are attracted to areas of high _____________ density

They form a ______________ bond using a pair of electrons from the bond

The most common electrophile is ________________

Addition reactions1. Reaction with hydrogen

Alkenes do not react with hydrogen under normal conditions…

Conditions:

2. Reaction with acidified potassium manganate(VII) This reaction involves addition across the double bond and oxidation

The products are alkane diols

Can be used as a test for _______________ as alkanes do not react

Conditions:

Observations:

3. Reactions with halogens Alkenes react halogens to form halogenoalkanes Reactions happen at room temperature and unlike the reaction of alkanes with halogens do

not involve ______ _____________

Reactions become __________ vigorous down the halogen group (F > Cl > Br > I)

4. Reaction with bromine water Bromine water is an _______________ solution of bromine

Addition takes place across the double bond and the bromine water becomes

_______________

There are two different products _____________________ and _____________________

2-bromoethanol is formed because _____ ions from water also take part in the reaction

5. Reactions with hydrogen halides Hydrogen halides are compounds containing ________________ and a halogen

e.g.

Hydrogen halides react with alkenes readily at room temperature

In the reactions hydrogen halides with higher alkenes (n > 2) there are two possible products

Both possible products are formed but one is favoured, in this case ___________________

The major product can be predicted using Markovnikov’s rule

Markovnikov’s rule:

“When HX adds across an asymmetric double bond, the major

product is the molecule in which hydrogen adds to the carbon

atom in the double bond with the greater number of hydrogen atoms already attached to it”

Reaction mechanisms – “Curly Arrows”

Key words

Electrophile –

Nucleophile –

Carbocation –

Curly arrow –

Reaction mechanisms Mechanisms are diagrams that represent how organic chemistry reactions happen in terms of

the movement of electrons

A single-headed curly arrow shows…

A double-headed curly arrow shows…

Mechanism for the reaction of ethene with hydrogen bromide

Mechanism for the reaction of propene with hydrogen bromideRemember Markovnikov? There are two possible products:

1.

2.

Both mechanisms involve carbocations (careful there’s a mistake in your book!):

The carbocation in reaction ____ is more stable

This is because the __________ groups donate electrons to stabilise the positive charge

As a result the major product is ___________________

Mechanism for the addition of bromine to ethene

The bromine-bromine bond is partially polarised by the electron rich ___ bond

The first stage of the reaction is ________________ attack

This is followed by _______________ attack on the carbocation by Br-

Polymerisation – The most important reaction of the alkenes

Key wordsPolymer –

Monomer –

Synthesis of polymers Polymers are widely made in addition reactions

Addition occurs across a double or triple bond

Common polymers1. Poly(ethene) (polythene)

Formed from the polymerisation of ____________________

Comes in 2 forms __________ or ____________

Conditions of polymerisation reaction determine which form is produced:

1.1 Low-density polythene (LDPE)

Properties Conditions of polymerisation reaction

Diagram of structure

Uses

1.2 High-density polythene (HDPE)

Properties Conditions of polymerisation reaction

Diagram of structure

Uses

2. Poly(propene)

Properties Conditions of polymerisation reaction

Monomer

Polymer

3. Poly(chloroethene) (PVC)

Properties Conditions of polymerisation reaction

Monomer

Polymer

4. Poly(tetrafluoroethene) (PTFE / “Teflon”)

Properties Monomer

Polymer

Properties of polymers The physical (e.g. _________________ , __________________ and ________________) and

chemical properties of polymers depend on their molecular structure:

1. Average length of polymer chain Tensile strength

Melting temperature

2. Branching of the chain Tensile strength

Melting temperature

Density

3. Intermolecular forces between chains Tensile strength

Melting temperature

4. Cross-linking between chains Rigidity

Hardness

Flexibility

Melting temperature