functional group analysis, reactions & mechanisms

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FUNCTIONAL GROUP ANALYSIS,

REACTIONS & MECHANISMS

Keane Campbell

MSc; BSc; ASc

Created: November 12, 2012

Revised: September 19, 2013

Describe Selected Chemical Reactions of

Alkanes Alkanes are the least reactive class of organic compounds. They contain only C-C and C-H single bonds, which have high bond enthalpies.

Most useful reactions of alkanes take place under energetic or high temperature conditions. Alkane reactions often form mixtures of products that are difficult to separate.

The chemistry of alkanes is restricted to combustion and substitution reactions. Radicals are the only species reactive enough to overcome the high activation energy required to break the strong C-C and C-H bonds.

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Describe Selected Chemical Reactions of Alkanes Halogenation

Alkanes react with halogens (F2, Cl2, Br2, I2) to form alkyl halides via a mechanism known as free radical substitution.

Heat or light is usually needed to initiate this halogenation. The reaction of alkanes with chlorine or bromine proceeds at a moderate rate and is easily controlled. The reaction with fluorine is often too fast to control, while iodine reacts very slowly or not at all.

For example methane reacts with chlorine (Cl2) to form chloromethane, dichloromethane, trichloromethane and tetrachloromethane.

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Describe Selected Chemical Reactions of Alkanes

Cracking

This is used to convert higher-boiling fractions (larger

hydrocarbons) into smaller ones. There are two types of

cracking:

a) Catalytic hydrocracking

b) Catalytic cracking

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Describe Selected Chemical Reactions of Alkanes

Combustion

This is a rapid oxidation that takes place at high temperatures,

converting alkanes to carbon dioxide and water.

Explain the steps involved in the mechanism of free

radical substitution Students, prior to examining the mechanism of free radical substitution, there are certain important concepts that we have to become cognizant of.

The bond-dissociation energy is the amount of energy required to break a particular bond homolytically, i.e., in such a way that each bonded atom retains one of the two electrons.

In contrast, when a bond is broken and one of the atoms retains both electrons, we say that heterolytic cleavage has occurred.

Homolytic cleavage (radical cleavage) forms free radicals, whereas heterolytic cleavage form ions. A heterolytic cleavage is sometimes called an ionic cleavage.

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Explain the steps involved in the mechanism of free

radical substitution Note that a curved arrow ( ) is used to show the movement of the electron pair in an ionic cleavage, and

Fish hook notation i.e. half arrows to show the separation of individual electrons in a homolytic cleavage.

The curved arrow begins with a covalent bond or unshared electron pair (a site of higher electron density) and points toward a site of electron deficiency.

Remember, the curved arrow shows the movement of electrons, while the fish hook shows the movement of one electron per atom i.e. separation.

Now, students, with all this said, what do we really mean when we speak of mechanism.

The mechanism is the complete, step-by-step description of exactly which bonds break and which bonds form in what order to give the observed products.

Explain the steps involved in the mechanism of free

radical substitution

The Free Radical Chain Reaction

The free radical substitution is also known as the free radical chain reaction.

A chain reaction consists of three kinds of steps:

1. The initiation step, which generates a reactive intermediate.

2. Propagation steps, in which the reactive intermediate reacts with a stable molecule to form another reactive intermediate, allowing the chain to continue until the supply of reactants is exhausted or the reactive intermediate is destroyed.

3. Termination steps, side reactions that destroy reactive intermediates and tend to slow or stop the reaction.

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Describe Selected Reactions of Alkenes

All alkenes have a functional group: a carbon-carbon double

bond. The reactions of alkenes arise from the reactivity of the

carbon-carbon double bond.

The most common chemical reaction of a carbon-carbon double

bond is the addition reaction. Because single bonds (sigma

bonds) are more stable than pi bonds, we might expect the

double bond to react and transform the pi bond into a sigma

bond.

Let us now look at the reactions of alkenes:

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Describe Selected Reactions of Alkenes 1. Bromination

a) Alkenes react with bromine in the presence of an organic solvent (CCl4) to form 1,2-dibromoalkane.

Note that when bromine in the presence of an organic solvent it is liquid bromine i.e. Br2 (l)

Note the organic solvent is inert, thus it does not participate in the reaction.

b) In the presence of water, halogens add to alkenes to form halohydrins i.e. a halo alcohol. In this case, molecules of the solvent become reactants, too:

Note that when bromine reacts with an alkene in the presence of water it is aqueous bromine i.e. Br2 (aq).

Electrophilic Addition of Bromine

Electrophiles are species that are electron-pair acceptors. They are

typically positive ions like H+ or NO2+.

Electrophiles accept electron pairs to form new bonds. The bromine is a

very molecule and the approaching pi bond in a molecule

i.e. there will be a partial positive and a partial negative charge on each

of the bromine atom, causing it to look like this:

The mechanism for the electrophilic addition of bromine to alkene occurs

in two steps:

Explains the steps involved in the mechanism of selected chemical

reactions of alkene functional group

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Describe Selected Reactions of Alkenes

2. Oxidation

a) Alkenes react with warm acidified potassium manganate (VII) solution to

form mixtures of ketones and carboxylic acids.

To be even more specific, students, warm concentrated KMnO4 oxidizes

alkenes to glycols, then cleaves the glycols. The products are initially

ketones and aldehydes, but aldehydes are oxidized to carboxylic acids

under these conditions.

b) Alkenes react with cold KMnO4. If the KMnO4 solution is acidified with

dilute H2SO4, the purple solution becomes colourless.

The manganate (VII) ions are reduced to manganese (II) ions while the

alkenes are converted to 1,2-diols.

Describe Selected Reactions of Alkenes

3. Concentrated Sulphuric Acid

When conc. H2SO4 adds across a carbon-carbon double bond an alkyl

hydrogen sulphate is produced.

When ethene is bubbled into conc. H2SO4 at room temperature, ethyl

hydrogen sulphate is formed:

The structure of the product molecule is sometimes written as

CH3CH2H2SO4, but the version in the equation is better because it

shows how all the atoms are linked:

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Describe Selected Reactions of Alkenes 4. Hydrogen halides

The hydrogen halides, HCl, HBr, and HI, all add across the double bond. When hydrogen bromide adds to ethene, bromoethane is formed:

When hydrogen halides add to an unsymmetrical alkene two products are possible. The addition of hydrogen bromide to prepare could give either

In fact, the product is almost entirely 2-bromopropane. The Russian chemist, Markovnikov, formulated a rule for predicting which addition product would be formed.

Rule can be stated: in addition of a compound HX to an unsaturated carbon atom which carries the larger number of hydrogen atoms.

Describe Selected Reactions of Alkenes

Electrophilic addition of hydrogen bromide

A molecule of hydrogen halide is permanently polarised.

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Describe Selected Reactions of Alkenes 4. Hydrogenation

This is the process by which hydrogen is added across a double bond at high temperature in the presence of finely nickel, platinum or palladium catalyst.

The process of hydrogenation is intended to add hydrogen atoms to cis-unsaturated fats, eliminating a double bond and making them more saturated. These saturated fats have a higher melting point, which makes them attractive for baking and extends their shelf-life.

However, the process frequently has a side effect that turns some cis-isomers into trans-unsaturated fats instead of hydrogenating them completely. A natural, unsaturated fatty acid might look like the molecule below. It has several double-bonds between adjacent carbon atoms, which is what makes it (saturated fats have no double bonds and all the available are taken up by hydrogen atoms.

When this oil is hydrogenated, it is not possible to control where the hydrogen atoms are added to the structure. If both hydrogen atoms are added to the same side of the structure. If both hydrogen atoms are added to the same side of the structure, it is called a fat.

Describe Selected Reactions of Alkenes

4. Hydrogenation

Cis fats exist naturally and, because the hydrogen atoms are crowded on

one side of the molecule, they bend, allowing other chemicals and

enzymes to bind to them.

If, however, one hydrogen atom adds to one side of the structure and the

other atom to the other side, it creates trans fats.

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4. Hydrogenation Trans fats do not exist naturally, with a very few exceptions. Because the structure is uncrowded, they do not bend and so other molecules and enzymes find it more difficult to bind them.

The shape of the molecule is therefore vital to its function, much in the same way as the shape of a key is important for operation of a lock.

Trans fats are straight and can pack into a crystal formation, which allows them to solidify at room temperature.

Trans fats have harmful effects on blood lipids, promote inflammation, and cause blood vessel abnormalities, all of which are risk factors for heart disease.

Trans fat raises your low-density lipoproteins (LDL) cholesterol and lowers your (HDL) cholesterol.

Describe Selected Reactions of Alkenes

Graded Worksheet Answer all the questions below:

1. Draw and label the isomers of 3-hexene using both the cis-trans. [4 marks]

2. Write and name the structural formula for the products that form when ethene reacts with each of the following reagents. [4 marks] a) Br2 in H2O

b) Br2 in CCl4

3. Why do alkenes show geometrical isomerism, whereas alkanes do not? [2 marks]

4. Outline mechanism for the electrophilic addition of HBr to 2-methyl-1-butene, and name the product. [6 marks]

5. Name the following compounds: [6 marks] a) CH3CH=CHCH2CH2CH3 b) CH3C(CH3)=CHCH2CH(CH3)2 c) CH3CH2CH=CH(CH2)2CH3 d) CH3CHClCHCH2 e) CH3CHCH(CH2)2CH3 f) (CH3)2C=C(CH3)2

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Describe Selected Chemical Reactions of Alcohols

Alcohols contain the functional group OH attached to an alkyl group.

The carbon that the OH group is attached to is referred to as the carbinol carbon atom.

There are three main classes of alcohols:

Primary (10);

Secondary (20)

Tertiary (30)

The classification is based on the number of alkyl groups (R) attached to the carbinol carbon atom i.e. the carbon bearing the OH group:

in a primary alcohol, it has one R group and two hydrogen atoms;

in a secondary alcohol, it has two R groups and one hydrogen atom;

in a tertiary alcohol, it has three R groups.

Now students, based on these classifications of alcohols, to what class would methanol belong?

Describe Selected Chemical Reactions of Alcohols

The most typical reactions of alcohols involve

nucleophilic substitution reactions in which the

OH group is replaced by an incoming group

such as a halogen.

Nucleophiles are species that are electron-pair

donors. Nucleophiles include negatively

charged ions such as CN-, OH- or Cl-, as well

as molecules bearing lone pairs of electrons

like H2O or NH3.

Nucleophiles attack the partially positively

charged ( +) atoms of polar covalent bonds.

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Describe Selected Chemical Reactions of Alcohols

Oxidation

Primary alcohols are oxidized to aldehydes and to carboxylic acids.

Secondary alcohols are oxidized to ketones.

Tertiary alcohols are resistant to oxidation. A powerful acidic oxidizing agent

converts them into a mixture of carboxylic acids.

A number of oxidizing agents can be used. Acidified K2Cr2O7 solution at room

temperature will oxidize primary alcohols to aldehydes and secondary alcohols to

ketones. At higher temperatures primary alcohols are oxidized further to acids.

The dichromate solution turns from the orange colour of Cr2O72- (aq) to the green

colour of Cr3+ (aq).

H+/KMnO4 solution. This is too powerful an oxidizing agent to stop at the aldehyde:

it oxidizes primary alcohols to carboxylic acids. It oxidizes secondary alcohols to

ketones.

Describe Selected Chemical Reactions of Alcohols

Esterification

Alcohols and carboxylic acids react to give esters. For example, ethanol and

ethanoic acid react in the presence of a concentrated sulphuric acid and catalyst to

form the ester ethyl ethanoate:

In this reaction, it is the C(O) OH bond in the acid that breaks and not the C OH

bond in the alcohol.

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Describe Selected Chemical Reactions of Alcohols

Dehydration Reactions

Dehydration is the removal of water. Dehydration requires an acidic catalyst to

protonate the hydroxyl group of the alcohol and convert it to a good leaving group.

Loss of water, followed by loss of a proton, gives the alkene. An equilibrium is

established between reactants and products.

According to the syllabus only equations are required for the reaction of alcohol with

carboxylic acid and conc. H2SO4.

Describe Selected Chemical Reactions of Alcohols

Dehydration Reactions

Now students, for teaching purposes only, I will show the mechanism so as to

enable better understanding of the aforementioned paragraphs.

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Describe Selected Chemical Reactions of Alcohols

Iodoform Test (I2, NaOH)

This is a positive test used to identify alcohols. When iodine and sodium hydroxide

are used, a positive reaction gives iodoform.

Iodoform (CHI3) is a pale yellow substance. Due to its high molar mass due to the

three iodine atoms, it is solid at room temperature. It is insoluble in water and has

an antiseptic smell. It is used as an antiseptic on the sort of sticky plasters you put

on minor cuts.

Iodine solution is added to a small amount of an alcohol, followed by just enough

sodium hydroxide solution to remove the colour of iodine. If nothing happens in the

cold it may be necessary to warm the mixture very gently.

A positive result is the appearance of a very pale yellow precipitate of

triiodomethane (previously known as iodoform) CH3.

Describe Selected Reactions of Halogenoalkanes Halogenoalkanes are classified depending on the number of alkyl groups attached to the carbon atom bonded to the halogen:

Primary (10) if there is one alkyl group attached to the carbon i.e. the carbon attached to the halide.

Secondary (20) if there are two alkyl groups attached to the carbon.

Tertiary (30) if there are three alkyl groups attached to the carbon.

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Describe Selected Reactions of Halogenoalkanes

Hydroxide ions hydrolyze halogenoalkanes to alcohols. Whenever a halogenoalkane is hydrolyze the corresponding alcohol is formed.

E.g. Bromoethane will form ethanol wherein the Br- is replaced by an OH ion.

The hydroxide ion is a strong nucleophile and the halide ion is a nucleophile.

Therefore we have one nucleophile replacing another nucleophile in the hydrolysis of halogenoalkanes which in essence is a nucleophilic substitution reaction.

Now students, whenever we have a nucleophilic substitution for a halogenoalkanes, there is bond breaking and bond making taking place.

Describe Selected Reactions of Halogenoalkanes Depending on the nature of the halogenoalkane, whether it be a primary or secondary alkyhalide (halogenoalkane), the bond breaking and making processes may occur in a

concerted manner or

Step-wise manner.

Now let us look at the word concerted. A concerted reaction is one in which both bond breaking and making are occurring at the same time.

Step-wise manner is, however, as it is suggested by its name, one process occurring before the other. i.e. bond breaking before bond forming.

For alkylhalide to have a concerted reaction it would have to be a primary alkylhalide.

The reason is simple students, a primary alkylhalide is unable to form a stable carbocation intermediate to await the entry of the (-OH) hydroxide ion.

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Hydrolysis of a Primary Halogenoalkane The reaction of ethyl iodide (bromoethane) with hydroxide ion is a concerted reaction, taking place in a single step with bonds breaking and making at the same time.

Hydroxide ion attacks the backside of the electrophilic carbon atom, donating a pair of electrons to form a new bond. (In general, nucleophiles are said to attack electrophiles, not the other way around.)

Notice that curved arrows are used to show the movement of electron pairs, from the electron-rich nucleophile to the carbon atom of the electrophile.

Hydrolysis of a Primary Halogenoalkane

Carbon can accommodate only eight electrons in its valence

shell, so the carbon-iodine bond must begin to break as the

carbon-oxygen bond begins to form.

The middle structure is a transition state, a point of maximum

energy, rather than an intermediate.

In this transition state, the bond to the nucleophile (hydroxide) is

partially formed and the bond to the leaving group (iodide) is

partially broken.

Note students, transition state is not a discrete molecule that can

be isolated; it only exists for an instant.

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Hydrolysis of a Tertiary Halogenoalkane The reaction of 2-bromo-2-methylpropane (t-butyl bromide) with hydroxide ion occurs in a step-wise manner forming an intermediate in the process.

Step 1: Formation of carbocation (rate-determining)

Step 2: Nucleophilic attack on the carbocation

Note the long arrow is suggesting that reaction is more stable on the reactant side. The first step is a slow ionization to form a carbocation. The second step is a fast attack on the carbocation by a nucleophile.

Describe Selected Chemical Reactions of

Carbonyl Compounds The carbonyl group consists of an oxygen atom attached to a

carbon atom by a double covalent bond, usually represented

by the formula

Aldehydes and ketones are two classes of compound that both

contain the carbonyl group. Aldehydes have the structure (as

given above) where R is an alkyl or aryl group.

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2, 4

reagent is used to test for the presence of carbonyl

compounds. If carbonyl compound is present: aliphatic

carbonyl compounds give orange or yellow precipitate,

while those of aromatic carbonyl compounds are darker in

colour.

A solution of 2,4-dinitrophenylhydrazine in methanol and

sulphuric acid is called reagent.

So formation of a yellow-orange precipitate of a DNP

derivative indicates the presence of an aldehyde or ketone.

Distinguishing Between Aldehydes & Ketones

Aldehydes have reducing properties whereas ketones do not have reducing properties.

There are two ways in which we can differentiate between an aldehyde and a ketone.

1) reagent: the test

reagents is sometimes called ammonical silver nitrate. It is a solution of complex silver ions, [Ag(NH3)2]

2+, is reduced to a silver mirror when warmed with aldehydes but not ketones.

2) solution

solution contains complex copper (II) ions. On warming, aldehydes reduce solution, which is blue, to a brick- red precipitate of copper (I) oxide Cu2O. (Methanal reduces to solution further to metallic copper.)

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Distinguishing Between Aldehydes & Ketones

Oxidation [KMnO4 (aq)/H+(aq)]

Aldehydes are readily oxidized to carboxylic acids by acidified potassium manganate (VII). Ketones are more difficult to oxidize than aldehydes. With a powerful oxidizing agent, in acid conditions, they are oxidized to a mixture of carboxylic acids.

Reduction of carbonyl compounds

Aldehydes are reduced to primary alcohols by

H2/(Pt or Ni)

Hydrogen adds across the carbonyl group, reducing aldehydes to primary alcohols and ketones to secondary alcohols. The reaction is more difficult than addition to a carbon-carbon double bond, and usually requires heat, pressure and a metal catalyst (Pt or Ni).

Distinguishing Between Aldehydes & Ketones Reduction of carbonyl compounds

Aldehydes are reduced to primary alcohols by

Lithium tetrahydridoaluminate, LiAlH4

LiAlH4 is used in solution in ethoxyethane to reduce aldehydes to primary alcohols and ketones to secondary alcohols.

So remember students:

Aldehydes are reduced to 10 alcohols by H2 in the presence of Pt or Ni and by LiAlH4 in ether.

Ketones are reduced by H2 in Pt or by LiAlH4 in ether to give 20 alcohols.

Cyanohydrin (Hydroxynitirile) Formation (NaCN/HCl)

A cyanohydrin reaction is an organic chemical reaction by an aldehyde or ketone with a cyanide anion to form cyanohydrin. The CN- is a nucleophile which attacks the carbonyl group causing it to become polarize and picking the H+ of HCl.

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Mechanism of Selected Chemical Reactions of

Carbonyl Compounds

Mechanism of Selected Chemical Reactions of

Carbonyl Compounds

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Selected Chemical Reaction of Carboxylic Acids

Salt Formation

Carboxylic acids form salts by reaction with metals, carbonates, hydrogen carbonates and alkalis:

RCOOH (aq) + NaOH (aq) RCOO- Na+ (aq)+ H2O (l)

HCOOH (aq) + NaOH (aq) HCOO-Na+ (aq) + H2O (l)

RCOOH (aq) + NaHCO3 (aq) RCOO- Na+ (aq) + H2O (l) + CO2 (g)

CH3COOH (aq) + NaHCO3 (aq) CH3COO-Na+ (aq) + H2O (l) + CO2 (g)

2RCOOH (aq) + Mg (s) (RCOO- )2Mg2+ (aq)+ H2 (g)

C3H7COOH (aq) + Mg (s) (C3H7COO-)2Mg

2+ (aq) + H2 (g)

The evolution of CO2 from sodium hydrogen carbonate is used as a test to distinguish carboxylic acids from weaker acids, such as phenols.

Selected Chemical Reaction of Carboxylic Acids

Esterification

Carboxylic acids react with alcohols in the presence of concentrated sulphuric acid to form esters:

The alkyl group after the ester linkage is written first.

Conversion to Acid Chlorides

Carboxylic acids react with SOCl2 (thionyl chloride), PCl3 (phosphorus trichloride) and PCl5 (phosphorus pentachloride) to produce acyl chlorides.

The conversion of carboxylic acids to acyl chlorides using thionyl chlorides (SOCl2) is more convenient than PCl3 and PCl5 since the by-products formed, using thionyl chloride are gases.

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Selected Chemical Reaction of Esters Esters contain the functional group:

where R and R1 may be alkyl groups and may be the same or different.

Ester hydrolysis

The hydrolysis of an ester gives an equilibrium mixture of carboxylic acid and alcohol. The attachment of equilibrium is catalysed by the presence of mineral acids:

CH3CO2C2H5 (l) + H2O (l) CH3COOH (aq) + C2H5OH (aq)

Simply put students, in acid hydrolysis of water you end up with your starting materials (what you use to make the esters).

Basic Hydrolysis

Basic hydrolysis of esters, called saponification, literally means making of soap.

Transesterification: Biodiesel Production

Transesterification is the process of exchanging the organic of an ester with the organic group of an alcohol. These reactions are often catalysed by the addition of an acid or base.

Example

Transesterification: alcohol + ester different alcohol + different ester

Example

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Transesterification: Biodiesel Production

The name has been given to transesterified vegetable oil to

describe its use as a diesel fuel.

One of the first uses of transesterified vegetable oil (biodiesel vegetable oil

is an ester) was to power heavy duty vehicles in South Africa before World

War II.

Animal and plant fats and oils are typically made of triglycerides which are

esters. Ethanol and methanol are commonly used in transesterification

process.

Normally this reaction will proceed either exceedingly slowly or not at all.

Heat, as well as an acid or base are used to help the reaction proceed more

quickly.

Note, students, transesterification is an example of hydrolysis of esters.

Transesterification: Biodiesel Production

A catalyst and a base is used in the process. The mixture of

fatty acids is the biodiesel.