AH Chemistry - Organic Chemistry and Instrumental Analysis - (b) - Synthesis

When an organic reaction takes place, what happens?

When an organic reaction takes place, bonds in the reactant molecules are broken and bonds in the product molecules are made. The process of bond breaking is known as bond fission.

What are the two different types of bond fission?

The two different types of bond fission are:

• Homolytic

• Heterolytic

What is homolytic fission?

Homolytic fission:

• Results in the formation of two neutral radicals.

• Occurs when each atom retains one electron from the σ covalent bond and the bond breaks evenly.

• Normally occurs when non-polar covalent bonds are broken.

What do reactions involving homolytic fission tend to result in, and what does this make them unsuitable for?

Reactions involving homolytic fission tend to result in the formation of very complex mixtures of products, making them unsuitable for organic synthesis.

What is heterolytic fission?

Heterolytic fission:

• Results in the formation of two oppositely charged ions

• Occurs when one atom retains both electrons from the σ covalent bond and the bond breaks unevenly.

• Normally occurs when polar covalent bonds are broken.

What do reactions involving heterolytic fission tend to result in, and what does this make them better suited for?

Reactions involving heterolytic fission tend to result in far fewer products than reactions involving homolytic fission, and so are better suited for organic synthesis.

What does a single-headed arrow indicate?

A single-headed arrow indicates the movement of a single electron.

What does a double-headed arrow indicate?

A double-headed arrow indicates the movement of an electron pair. add pic

What does the tail of the arrow show?

The tail of the arrow shows the source of the electron(s).

What does the head of the arrow indicate?

The head of the arrow indicates the destination of the electron(s).

What do two single-headed arrows starting at the middle of a covalent bond indicate?

Two single-headed arrows starting at the middle of a covalent bond indicate that homolytic bond fission is occuring.

What does a double-headed arrow starting at the middle of a covalent bond indicate?

A double-headed arrow starting at the middle of a covalent bond indicates that heterolytic bond fission is occuring.

What does an arrow drawn with the head pointing the the space between two atoms indicate?

An arrow drawn with the head pointing to the space between two atoms indicates that a covalent bond will be formed between those two atoms.

In reactions involving heterolytic bond fission, how are attacking groups classified?

In reactions involving heterolytic bond fission, attacking groups are classified as nucleophiles or electrophiles.

What are nucleophiles?

Nucleophiles are:

• Negatively charged ions or neutral molecules that are electron rich, such as Cl^-, Br^-, OH^-, CN^-, NH₃, and H₂O

• Attracted towards atoms bearing a partial (δ⁺) or full positive charge.

• Capable of donating an electron pair to form a new covalent bond.

What are electrophiles?

Electrophiles are:

• Positively charged ions or neutral molecules that are electron deficient, such as H⁺, NO₂⁺, and SO₃

• Attracted towards atoms bearing a partial (δ^-) or a full negative charge.

• Capable of accepting an electron pair to form a new covalent bond.

What are haloalkanes?

Haloalkanes (alkyl halides) are substituted alkanes in which one or more of the hydrogen atoms is replaced with a halogen atom.

Describe monohaloalkanes

Monohaloalkanes:

• Contain only one halogen atom.

• Can be classified as primary, secondary, or tertiary according to the number of alkyl groups attached to the carbon atom containing the halogen atom.

• Take part in elimination reactions to form alkenes using a strong base, such as potassium or sodium hydroxide in ethanol.

• Take part in nucleophilic substitution reactions with:

- Aqueous alkalis to form alcohols

-Alcoholic alkoxides to form ethers

-Ethanolic cyanide to form nitriles (chain length increased by one carbon atom) that can be hydrolysed to carboxylic acids.

What are the two ways a monohaloalkane can take part in nucleophilic substitution reactions?

The two ways a monohaloalkane can take part in nucleophilic substitution reactions are:

• S_N1

• S_N2

What is S_N1?

S_N1 is a nucleophilic substitution reaction with one species in the rate determining step and occurs in a minimum of two steps via a trigonal planar carbocation intermediate.

What is S_N2?

S_N2 is a nucleophilic substitution reaction with two species in the rate determining step and occurs in a single step via a five-centred, trigonal bipyramidal transition state.

How can reaction mechanisms for S_N1 and S_N2 reactions be represented?

The reaction mechanisms for S_N1 and S_N2 reactions can be represented using curly arrows.

What is steric hinderance?

Steric hinderance and the inductive stabilisation of the carbocation intermediate can be used to explain which mechanism will be preferred for a given haloalkane.

What are alcohols?

Alcohols are substituted alkanes in which one or more of the hydrogen atom is replaced with a hydroxyl functional group, -OH group

How can alcohols be prepared?

Alcohols can be prepared from:

• Haloalkanes by substitution

• Alkenes by acid-catalysed hydration (addition)

• Aldehydes and ketones by reduction using a reducing agent such as lithium aluminium hydride.

What are the reactions of alcohols?

The reactions of alcohols include:

• Dehydration to form alkenes using aluminium oxide, concentrated sulfuric acid, or concentrated phosphoric acid.

• Oxidation of primary alcohols to form aldehydes and then carboxylic acids and secondary alcohols to form ketones, using acidified permanganate, acidified dichromate, or hot copper (II) oxide.

• Formation of alcoholic alkoxides by reaction with some reactive metals such as potassium or sodium, which can be reacted with monohaloalkanes to form ethers.

• Formation of esters by reaction with carboxylic acids using concentrated sulfuric acid or concentrated phosphoric acid as a catalyst.

• Formation of esters by reaction with acid chlorides (shown below). This gives a faster reaction than reaction with carboxylic acids, and no catalyst is needed. add pic

What do hydroxyl groups make alcohols?

Hydroxyl groups make alcohols polar, which gives rise to hydrogen bonding. This hydrogen bonding can be used to explain the properties of alcohols, including boiling points, melting points, viscosity, and solubility or miscibility in water.

What are ethers?

Ethers can be regarded as substituted alkanes in which a hydrogen atom is replaced with an alkoxy functional group, -OR, and have the general structure, R’ - O - R’’, where R’ and R’’ are alkyl groups. add pic of ether group

How can ethers be prepared?

Ethers can be prepared in a nucleophilic substitution reaction by reacting a monohaloalkane with an alkoxide.

Due to the lack of hydrogen bonding between ether molecules, what do they have?

Due to the lack of hydrogen bonding between ether molecules, they have lower boiling points than the corresponding isomeric alcohols.

Which ethers are soluble in water?

Methoxymethane and methoxyethane are soluble in water. Larger ethers are insoluble in water due to increased molecular size.

What are ethers commonly used as?

Ethers are commonly used as solvents since they are relatively inert chemically and will dissolve many organic compounds.

How can alkenes be prepared?

Alkenes can be prepared by:

• Dehydration of alcohols using aluminium oxide, concentrated sulfuric acid, or concentrated phosphoric acid.

• Base-induced elimination of hydrogen halides from monohaloalkanes.

What do alkenes take part in electrophilic addition reactions with?

Alkenes take part in electrophilic addition with:

• Hydrogen to form alkanes in the presence of a catalyst.

• Halogens to form dihaloalkanes.

• Hydrogen halides to form monohaloalkanes.

• Water using an acid catalyst to form alcohols.

What does Markovnikov’s rule state?

Markovnikov’s rule states that when a hydrogen halide or water is added to an unsymmetrical alkene, the hydrogen atom becomes attached to the carbon with the most hydrogen atoms attached to it already. Markovnikov’s rule can be used to predict major and minor products formed during the reaction of a hydrogen halide or water with alkenes.

How can the reaction mechanisms for the addition of a hydrogen halide and the acid-catalysed addition of water be represented?

The reaction mechanisms for the addition of a hydrogen halide and the acid-catalysed addition of water be represented using curly arrows and showing the intermediate carbocation. The inductive stabilisation of intermediate carbocations formed during these reactions can be used to explain the products formed.

How can the reaction mechanism for the addition of a halogen be represented?

The reaction mechanism for the addition of a halogen can be represented using curly arrows and showing the cyclic ion intermediate.

How can carboxylic acids be prepared?

Carboxylic acids can be prepared by:

• Oxidising primary alcohols using acidified permanganate, acidified dichromate, and hot copper (II) oxide.

• Oxidising aldehydes using acidified permanganate, acidified dichromate, Fehling’s solution, and Tollens’ reagent.

• Hydrolysing nitriles, esters, or amides.

What do reactions of carboxylic acids include?

Reactions of carboxylic acids include:

• Formation of salts by reactions with metals or bases.

• Condensation reactions with alcohols to form esters in the presence of concentrated sulfuric or concentrated phosphoric acid.

• Reaction with amines to form alkylammonium salts that form amides when heated.

• Reduction with lithium aluminium hydride to form primary alcohols.

What are amines?

Amines are organic derivatives of ammonia in which one more hydrogen atoms of ammonia has been replaced by an alkyl group.

What can amines be classified as?

Amines can be classified as primary, secondary, or tertiary according to the number of alkyl groups attached to nitrogen atom.

What do amines react with acids to form?

Amines react with acids to form salts.

Which amines display hydrogen bonding?

Primary and secondary amines, but not tertiary amines, display hydrogen bonding. As a result, primary and secondary amines have higher boiling points than isomeric tertiary amines.

What can primary, secondary, and tertiary amine molecules hydrogen-bond with?

Primary, secondary, and tertiary amine molecules can hydrogen-bond with water molecules, thus explaining the appreciable solubility of the shorter chain length amines in water.

Amines like ammonia are weak bases and do what?

Amines like ammonia are weak bases and dissociate to a slight extent in aqueous solution. The nitrogen atom has a lone pair of electrons which can accept a proton from water, producing hydroxide ions.

What is benzene?

Benzene (C₆H₆) is the simplest member of the class of aromatic hydrocarbons. The benzene ring has a distinctive structural formula.

What is the stability of the benzene ring due to?

The stability of the benzene ring is due to the delocalisation of electrons in the conjugated system. The presence of delocalised electrons explains why the benzene ring does not take part in addition reactions.

How can bonding in benzene be described?

Bonding in benzene can be described in terms of sp² hybridisation, sigma and pi bonds, and electron delocalisation.

What is a benzene ring which one hydrogen atom has been substituted by another group known as?

A benzene ring which one hydrogen atom has been substituted by another group is known as the phenyl group. The phenyl group has the formula C₆H₅.

What reactions can benzene rings take part in, and what do these reactions at benzene rings include?

Benzene rings can take part in electrophilic substitution reactions. Reactions at bezene rings include:

• Halogenation by reaction of halogen using aluminium chloride or iron (III) chloride for chlorination, and aluminium bromide or iron (III) bromide for bromination.

• Alkylation by reaction of a haloalkane using aluminium chloride.

• Nitration using concentrated sulfuric acid and concentrated nitric acid.

• Sulfonation using concentrated sulfuric acid.