Synthesis 1-3

Question 1

What is bond fission?

Answer 1

Bond fission is the process of breaking bonds and can either be homolytic of heterolytic.

Question 2

What is homolytic fission?

Answer 2

Homolytic fission is when the electrons from the bond are split equally between the atoms, and so usually occurs when non-polar covlent bonds are broken. This type of fission produces two free radicals with homolytic fission being the initiation step. Due to this reason, homolytic fission is often unsuitable for the synthesis of organic molecules.

Question 3

What are free radicals?

Answer 3

Free radicals are highly reactive species produced from homolytic fission (initiation). They react with many other species to produce more free radicals (propagation). Termination is the end step of this chain of reactions, which is where the free radicals react with eachother to form stable molecules.

Question 4

What is heterolytic fission?

Answer 4

Heterolytic fission is when the electrons from the bond are split unequally, and so usually occurs in polar covalent bonds. The atom with the higher electronegativity takes both of the electrons in the bond, becoming a negatively charged ion. The other atom becomes a positive ion as it loses an electron. This type of fission is better suited than homolytic fission for the synthesis of organic compounds.

Question 5

How are arrows used to represent the movement of electrons in:

a) homolytic fission
b) heterolytic fission

Answer 5

a) In homolytic fission, a single-headed curly arrow is used to indicate the movent of single electrons - one to each atom. The tail of the arrow indicates where the electron starts, and the head of the arrow indicates where the electron finishes.
b) In heterolytic fission, a double-headed curly arrow is used to indicate the movement of a pair of electrons - both electrons going to one atom.

Question 6

What is a nucleophile?

Answer 6

Nucleophilles are negatively charged ions or molecules that are electron rich. They donate a pair of electrons to an electrophille to form a covalent bond. The electrons in a nucleophille can either be non-bonded electron pairs (an ion) or bonded electron pairs (like in pi bonds).

Question 7

What is an electrophile?

Answer 7

An electrophille is a positively charged ion or a neutral molecule that is deficiant in electrons. They accept an electron pair from a nucleophille to form a covalent bond.

Question 8

What is a haloalkane?

Answer 8

Haloalkanes are alkanes where one or more of the hydrogen atoms have been replaced by a halogen. Haloalkanes with only one type of halogen in them are called monohaloalkanes.

Question 9

How do you name haloalkanes?

Answer 9

The same way you name other hydrocarbons; first you write the number of carbon atom the halogen is atached to, followed by a dash with the name of the halogen (it changes to end in ‘o’, e.g chloro) and then finally the name of the alkane. If there is more than one halogen, or it is a branched hydrocarbon, you name them in alphabetical order.

Question 10

How do you know if a monohaloalkane has a primary, secondary, or tertiary structure?

Answer 10

Monohaloalkanes are classified depending on the number of alkyl groups (e.g CH3) attached to the carbon atom that the halogen is attached to. If there is none or only one alkyl group attached, it is a primary structure, if there are two attached it has a secondary structure and if there are three, it is a tertiary structure.

Question 11

What are the main reactions that monohaloalkanes undergo?

Answer 11

Elimination reactions and nucleophilic substitution reactions.

Question 12

What is an elimination reaction?

Answer 12

When aroms or small molecules are removed from the reactant molecule and not replaced. For example, making alkenes from monohaloalkanes.

Question 13

How do monohaloalkanes undergo elimination reactions (also called base-induced elimination of hydrogen halides from monohaloalkanes) to produce alkenes?

Answer 13

First the monohaloalkane is heated under reflux with a solution of potassium or sodium hydroxide dissolved ethanol. The nucleophilic OH ion attacks the H ion adjacent to the halogen atom, forming a water molecule. At the same time, the pair of electrons from the C-H bond move to form a double bond between the two carbon atoms. Finally, the Carbon to halogen bond breaks heterolytically, releasing a halogen ion and completing the double carbon to carbon bond. . The halogen and hydrogen atoms have been eliminated from the haloalkane and not replaced. This recation is also called a base-induced elimination reaction.

Question 14

Why can monohaloalkanes undergo nucleophilic substitution reactions?

Answer 14

Because the slight positive charge on the carbon atom makes it act as an electrophile so it is suspectiple to be attacked by nucleophiles. The nucelophille donates an electron pair to the haloalkane to form a covalent bond, and at the same time the halogen is removed as a halide ion. The halogen is substituted by the nucleophille.

Question 15

What is a substitution reaction?

Answer 15

A substitution reaction is where an atom or group of atoms in a molecule is replaced by another atom or group of atoms.

Question 16

What can monohaloalkanes undergo nucleophilic substitution reactions?

Answer 16

• Aqueous alkalis (e.g. KOH, NaOH) to form alcohols.
• Alcoholic alkoxides (e.g. potassium methoxide in methanol) to form ethers.
• Ethanolic potassium or sodium cyanide to form nitriles.

Question 17

How are alkoxides formed?

Answer 17

Alkoxides (such as the alkoxide potassium methoxide) are formed when an alkali metal is added to an alcohol. (e.g. when potassium is added to methanol, potassium methoxide is formed).

Question 18

What is a nitrile?

Answer 18

A nitrile is an organic compound which contains a Nitrogen atom bonded to one of the carbon atoms (contains a CN group). When naming nitriles, the prefix (methyl, ethyl etc) must reflect the total number of carbon atoms, including the one in the CN group. Nitriles can be converted into a carboxylic acid by acid hydrolysis (reation with water and hydrogen ions from an acid).

Question 19

What happens in an Sn1 nucleophilic substitution mechanism?

Answer 19

An Sn1 reaction happens in two steps. It follows first order kinetics, which means only one species is involved in the rate-determining step. A carbocation intermediate is formed (a carbon that has lost an electron pair so is bonded to only 3 things).

Question 20

Describe the Sn1 recation between 2-bromo-2-methylpropane and a hydroxide ion.

Answer 20

1) The first step is the slow rate-determining step, where the haloalkane undergoes heterolytic fission as the C-Br bond breaks to form a carbocation intermediate and a Bromine ion. When the halogen breaks of the carbocation will form a new shape to minimise repulsion.
2) This is the fast-step and involves nucleophilic attack of the hydroxide ion on the carbocation which can happen from either side.

Question 21

What happens in an Sn2 nucleophiliuc substitution reaction?

Answer 21

An Sn2 reaction happens in one continous step. It follows second-order kinetics, which means two species are involved in the rate-determining step. A transition state/five-centred transition state is reached in the middle of the reaction, where there is a five-bonded carbon with two of the bonds partially formed and partially broken.

Question 22

Describe the Sn2 reaction between bromoethane and a hydroxide ion.

Answer 22

The nucleophillic hydroxide ion attacks the slighlty positively charged carbon atom (due to carbon having lower electronegativity than halogen) from the side opposite the C-Br bond. A covalent bond begins to form between thr C-OH and at the same time the bond between the C-Br begins to break, forming the five-centred transition state. The transition state forms a trigonal bipyramidal structure to minimise repulsion, with the two C-H bonds and the C-C bonds in a plane and the two partial bonds lying perpidicular to this plane. Finally, a complete covalent bond is formed between the C-OH and the Bromine ion is kicked out.

Question 23

How do you tell if a reaction will be Sn1 or Sn2?

Answer 23

Whether a haloalkane undergoes a Sn1 or Sn2 depends on its structure.

• Alkyl groups have an inductive effect on the central slightly positive carbon atom in a carbocation. This is because alkyl groups are electron-donating and push their electrons into the Carbon atom, which stabilises a carbocation. Therefore, tertiary carbocations will be the most stable and primary ones the least stable. This means that tetrtiary haloalkanes are most likely to react via Sn1.
• The size of the alkyl groups also have an effect known as the steric effect. If a tertiary haloalkane was to undergo an Sn2 reaction, it would be difficult for the nucleophille to attack the haloalkane with 3 bulky alkyl groups in the way, meaning tertairy haloalkanes are unlikely to undergo an Sn2 reaction. Primary haloalkanes only have one alkyl group so the nucleophille would be able to react with them, meaning primary haloalkanes are more likley to undergo an Sn1 reaction.