Topic 10.6: Reaction pathways

10.6.1 Deduce reaction pathways given the starting materials and the product

Alkene --> Ketone

Alkane --> Alcohol

Halogenalkane --> Aldehyde

Every arrow on the picture represents a reaction. IB requires you to remember all reaction represented

Alkene

--> Polyalkene

+multiple alkene

Polymerisation reaction

--> 1,2 - Dihalogenoalkane

+ halogen

Halogenation reaction

--> Halogenoalkane

+ Hydrogen halide

--> Alkanes

+ Hydrogen (Nickel catalyst, heat)

Hydrogenation reaction

--> Alcohols

+ Water (Concentrated sulphuric acid catalyst)

Hydration reaction

Alkane

--> Halogenoalkane

+ Halogens (UV light catalyst)

Free radical mechanism required

Halogenoalkane

--> Dihalogenoalkane

+ Halogen (UV light catalyst)

--> Alcohol

+ Aqueous hydroxide (warm)

Substitution reactions

1st degree Alcohol

--> Aldehyde

+ Heat and potassium dichromate catalyst

Oxidation reaction

2nd degree Alcohol

--> Ketone

+ Heat and potassium dichromate catalyst

Oxidation reaction

Aldehyde

--> Carboxylic acid

+ Heat and potassium dichromate catalyst

Oxidation reaction

Topic 10.5: Halogenoalkanes

10.5.1 Describe, using equations, the substitution reactions of halogenoalkanes with sodium hydroxide

Halogenoalkanes can be classified according to their structure as primary, secondary or tertiary.

This class of structure plays an important role in the reactions of the halogenoalkanes. Halogenoalkaenes contain polar carbon - halogen bonds that make them easy to convert to other products

Halogenoalkanes will undergo nucleophilic substitution (Sn) reactions. This involves the attack of a negatively charged / neutral electron rich species (called a nucleophile) on the slightly positive carbon atom bonded to the halogen

This causes the polar carbon - halogen bond to break so that the halogen is substituted by the nucleophile

The are two mechanisms for nucleophilic substitution reactions, called SN1 and SN2. This stands for substitution nucleophilic first or second order.

10.5.2 Explain the substitution reactions of halogenoalkanes with sodium hydroxide in terms of SN1 and SN2 mechanisms

Tertiary halogenoalkanes react in a SN1 mechanism

The rate of this reaction is dictated by one factor, the concentration of the halogenoalkane

Only the halogenoalkane is involved in the rate expression and therefore involved in the rate determining step

This is therefore a first order reaction that is unimolecular process with a molecularity of one

The first stage of the mechanism involves the breaking of the carbon - halogen bond to give an intermediate carbocation

This is bond breaking is called heterolytic fission. This is a slow process and so is the rate-determining step

The second stage involves the nucleophile bonding with the carbocation intermediate. This happens very quickly. It therefore does not appear in the rate expression

Note: To gain all the marks off this questions, this is directly pulled off the answers of past papers

SN1:
Curly arrow showing Cl leaving;
Representation of tertiary carbocation;
curly arrow going from lone pair / negative charge on O in OH- to C+;
Do not allow arrow orginating on H in OH-;
Formation of organic product
(CH3)3COH and CL-


SN2 mechanism has primary halogenoalkanes reacting this way.

The rate of reaction is dictated by two factors. By both the concentration of the halogenoalkane and the concentration of the nucleophile

Both the halogenoalkane and the nucleophile are involved in the rate expression and therefore both are involved in the rate determining step

This is a second order reaction, bimolecular process with a molecularity of two.

This mechanism involves the simultaneous attack of the nucleophile and loss of the halogen

An activated complex is formed during the process

Note: To gain all the marks off this questions, this is directly pulled off the answers of past papers

SN2:
Curly arrow going from lone pair / negative charge on O in OH- to C;
Do not allow curly arrow originating on H in OH-:
Curly arrow showing Br leaving;
Accept curly arrow either going from bond between C and Br to Br in bromoethane or in the transition state;
Representation of transition state showing negative charge, square bracket and partial bonds;
Do not penalize if HO and BR are not a t 180 to each other
Do not award M3 if OH---C bond is represented
Formation of organic product CH3CH2OH and Br-

Topic 10.4: Alcohols

10.4.1 Describe, using equations, the complete combustion of alcohols

The complete combustion of alcohols produces water and carbon dioxide. It is extremely important to remember the alcohols already have one oxygen atom in them already.

The equation should look something like this.

10.4.2 Describe, using equations, the oxidation reactions of alcohols

Oxidation reactions of alcohol require heating with an acidic solution of potassium dichromate (IV)

There are three types of alcohols depending on their degree of carbon, they will undergo different reactions.


Primary alcohols are oxidised to aldehydes, with the alcohol group becoming a carbonyl group

Repeating the same process for aldehydes would create carboxylic acids.

Secondary alcohols are oxidised to ketones. The ketones are unable to oxidise further.

Tertiary alcohols are unable to be oxidised.

10.4.3 Determine the products formed by the oxidation of primary and secondary alcohols

The products formed by the oxidation of primary alcohols are aldehydes then carboxylic acids.

The products formed by the oxidation of secondary alchols are ketones.

Topic 10.3: Alkenes

10.3.1 Describe, using equations, the reactions of alkenes with hydrogen and halogens

With hydrogen

Hydrogen reacts with alkenes to form alkanes in the presence of a nickel catalyst at about 150 degree celcius.

With halogens

Halogens react with alkenes to produce dihalogeno compounds. These reactions happen quickly at room temperature and are accompanied by the loss of colour of the reacting halogen.

10.3.2 Describe, using equations, the reactions of symmetrical alkenes with hydrogen halides and water

With hydrogen halides

Hydrogen halides react with alkenes to produce halogenoalkanes. These reactions take place rapidly in solution at room temperature

With water

The reaction with water is known as hydration and converts the alkene into an alcohol. It can be achieved by using concentrated sulphuric acid as a catalyst. The reaction involves an intermediate in which both hydrogen ion and hydrogen sulphide  ions are added across the double bond. This is quickly followed by a hydrolysis with a replacement of the hydrogen sulphide ion by a hydroxide ion.

10.3.3 Distinguish between alkanes and alkenes using bromine water

We can use the fact that alkenes readily undergo addition reactions, whereas alkanes will not (and will only undergo substituion reactions in UV light), as the basis of tests to distinguish between members of the two homologous series.

Alkanes have no reaction with bromine water, thus the solutions remain brown. Alkenes react with bromine water causing it to become colour because bromoalkanes are colourless.


10.3.4 Outline the polymerization of alkenes

Due to the fact that alkenes undergo addition reactions by breaking their double bonds, they can be joined together to produce long chain known as polymers. The alkene used in this reaction is known as the monomer and its chemical nature will determine the properties of the polymer.

Ethene polymerizes to form polyethene.

10.3.5 Outline the economic importance of the reactions of alkenes

Polychloroethene is also known as PVC and is widely used in all forms of construction materials, packaging, electrical cable sheathing and so on. It is one of the world's most important plastics. Its widespread use is, however, somewhat controversial as its synthesis is associated with some toxic-by products

Topic 10.2: Alkanes

10.2.1 Explain the low reactivity of alkanes in terms of bond enthalpies and bond polarity

Alkanes contain only carbon - carbon and carbon - hydrogen bonds. These are both strong bonds so these molecules will only react in the presence of strong source of energy, strong enough to break these bonds. As a result, alkanes are stable under most conditions and can be stored, transported and even compressed safely.

Both carbon - carbon and carbon - hydrogen bonds are also charateristically non-polar, so these molecules are not susceptible to attack by most common reactants. These two factors taken together mean that alkanes are generally of very low reactivity.

10.2.2 Describe, using equations, the complete and incomplete combustion of alkanes

Combustion are highly exothermic. This is mainly because of the large amount of energy released in forming the double bonds in carbon dioxide and the bonds in water.

Alkanes burn in the presence of excess oxygen produces carbon dioxide and water. When oxygen supply is limited, carbon monoxide is produced instead of carbon dioxide.

10.2.3 Describe, using equations, the reactions of methane and ethane with chlorine and bromine

Substitution reactions of alkanes are known as halogenalkanes. The reaction cannot take place in the dark as the energy of UV light is necessary to break the covalent bond in the chlorine molecule. This splits the chlorine molecule into chlorine atoms, which each have an unpaired electron and are known as free radical.

10.2.4 Explain the reactions of methane and ethane with chlorine and bromine in terms of a free-radical mechanism

Free-radical mechanism has three parts, this includes initiation, propagation and termination.

Initiation

This process occurs in the presence of ultra-violet light. It is known as homolytic fission because the bonds between the chlorine atoms is broken, splitting the shared pair of electrons between the two atoms.


Propagation

These reactions are called propagation because they both use and produce free radicals, and so allow the reaction to continue. This is why the reaction is often called a chain reaction.


Termination

These reactions remove free radicals from the mixture by cause them to react together and pair up their electrons.

Topic 10.1: Introduction

10.1.1 Describe the features of a homologous series

Homologous series is the name given to a group of compounds that can be described by a general formula

• All members of a homologous series have similar chemical properties
• The next member in a homologous series deiffers by a CH2 group (called a repeating unit)
• This causes a gradual change in physical properties as the chain length increases.

10.1.2 Predict and explain the trends in boiling points of members of a homologous series

Boiling point increases as chain length increases. This is due to an increase in Van De Waal forces between the molecules as the molar mass and surface area increases.

10.1.3 Distinguish between empirical, molecular and structural formulas

Organic structures can be represented in a variety of ways:

Empirical Formula is the simplest ratio of atoms of each elements in a compound

Molecular Formula is the actual number of atoms of each element presented in the compound

Full structural formula shows every bond and atom. Usually 90 and 180 degrees angles are used to show the bonds because this is the clearest representation on a 2-dimensional page,

Condensed structural formula often omits bonds where they can be assumed, and group atoms together. It contains the minimum information needed to describe the molecule non-ambiguously - in other words there is only one possible structure that could be described by this formula.

10.1.4 Describe structural isomers as compounds with the same molecular formula but with different arrangements of atoms

Structural isomers are different arrangements of the same atoms but make different molecules. Each isomer is a distinct compound, having unique physical and chemical properties.

10.1.5 Deduce structural formulas for the isomers of the non-cyclic alkanes up to (6 carbons) C6

Read the name of the isomer, and deduce the shape using the knowledge gained from 10.1.6

10.1.6 Apply IUPAC rules for naming the isomers of the non-cyclic alkanes up to (6 carbons) C6

Identify the longest carbon atom chain, it becomes the stem of the name.

1 - meth
2 - eth
3 - prop
4 - but
5 - pent
6 - hex

Side chains are placed in the suffix (-ane), are known as substituents. The number in front shows the number of the carbon it is linked to.

methyl, ethyl, propyl...

10.1.7 Deduce structural formulas for the isomers of the straight-chain alkenes up to (6 carbons) C6

Read the name of the isomer, and deduce the shape using the knowledge gained from 10.1.8

10.1.8 Apply IUPAC rules for naming the isomers of the straight-chain alkenes up to (6 carbons ) C6

Identify the longest carbon atom chain, it becomes the stem of the name.

1 - meth
2 - eth
3 - prop
4 - but
5 - pent
6 - hex

Side chains are placed in the suffix (-ene), are known as substituents. The number in front shows the number of the carbon it is linked to.

The number in the middle signifies where the double bond is. Trans and Cis show how the ethene is connected.

10.1.9 Deduce structural formulas for compounds containing up to six carbon atoms with one of the following functional groups: alcohol, aldehyde, ketone, carboxylic acid and halide

This will be discussed in 10.1.10

10.1.10 Apply IUPAC rules for naming compounds containing up to six carbon atoms with one of the following functional groups: alcohol, aldehyde, ketone, carboxylic acid and halide

Similar to previous syllabus statements, alcohol's suffix is (-anol), aldehyde's suffix is (-anal), ketone's suffix is (-anone), carboxylic acid's suffix is (-anoic acid). Halide's have a prefix dependent on the type of halogen. (Fluoro-, Chloro-, Bromo-, Iodo-)


Alcohol

Aldehyde

Ketone

Carboxylic acid

Halide

10.1.11 Identify the following functional group when present in structural formulas: amino (NH2), benzene ring and esters (RCOOR)

When looking at the structural formula double check for any amino, benzene ring and esters

10.1.12 Identify primary, secondary and tertiary carbon atoms in alcohols and halogenoalkanes

Identifying what degree carbon depends on how many other carbons it is connected to.

10.1.13 Discuss the volatility and solubility in water of compounds containing the functional groups listed in 10.1.9

Volatility is a measure of how easily a substance changes into the gaseous state - high volatility means that the compound has a low boiling point. Volatility is dependent on overcoming the forces between the molecules, so the stronger the intermolecular forces, the higher the boiling point.

Most volatile

Alkane
Halogenalkane
Aldehyde
Ketone
Alcohol
Carboxylic acid

Least volatile

Solubility is largely determined by the extent to which the solute molecules are able to interact and form hydrogen bonds with water.