IB SL Chemistry - Reactivity 3.3 + 3.4

Radical

An atom or polyatomic species that has an unpaired electron.

True or False: Radicals can not exist independently.

False- they can exist independently. (Radicals are different from charged species, like ions.)

Are radicals the ultimate products in a reaction?

No, radicals are highly reactive so they are not the final products.

What happens during termination and what step in the process of a radical substitution reaction is it?

Termination is when two radicals react together to form a new covalently bonded compound with no unpaired electrons. They are more stable and are the final products. This is the final step in the reaction.

What happens during propagation and what step in the process of a radical substitution reaction is it?

Propagation is a cycle that occurs where a radical reacts with a non radical to create further radical species. This is the second step in the reaction, but can occur multiple times before the final step.

What happens during initiation and what step in the process of a radical substitution reaction is it?

Homolytic fission- Note: the reactants are not radicals but they produce two radicals.

What is homolytic fission and how does it occur?

The process a molecule undergoes to become a radical. (Occurs during initiation.) Must be exposed to UV light or heat to excite the bonds.

What are substitution reactions?

The replacement of an atom or a group of atoms in an organic molecule with another atom/group of atoms.

Why might one want to transform an alkane into a more reactive species through halogenation?

Due to the fact that radicals are relatively inert (because of the strength of their C-C and C-H bonds), their bonds are non-polar, which makes them unreactive. However, through halogenation using radical substitution reactions, the new polar bonds in an alkane can be used in other organic chemistry reactions.

True or False: Halogenation of an alkane tends to occur with all Halogens (F, Cl, Br and I).

False- halogenation tends to only occur with Cl and Br as F is too reactive and I is too low.

Nucleophile

An electron-rich species that contains a lone pair of electrons. Can be neutral (with partial charges) or negatively charged. (Reactant)

Electrophile

An electron-deficient species that readily accepts a pair of electrons. Can be neutral (with partial charges) or positively charged.

kind of bond forms between a nucleophile and an electrophile?

A covalent coordination bond.

What are the two examples of a nucleophile relating to water?

1) H₂O has two lone pair and has no charge.

2) OH^- has three lone pairs and is negatively charged.

What does strength of a nucleophile depend on?

Strength depends on its ability to donate its electron pair to an electrophile.

Nucleophile substitution reactions

Where the nucleophile donates an electron pair to the electrophile, forming a bond. But this also results in the breaking of one of the bonds in the electrophile, producing a small molecule (or leaving group).

General equation: Nu^- + R-X → R-Nu + X^-

Heterolytic fission

When the electrons in a bonding pair are distributed unevenly, meaning that one atom receives both bonding electrons (forming an anion) and the other atom does not receive any bonding electrons (forming a cation).

What do halogenoalkanes form when they undergo heterolytic fission?

An alkyl cation and a halogen anion.

What are carbocations?

Alkyl cations with the positive charge on the carbon atom

True or False: The two species formed during heterolytic fission have a short life span and therefore are usually intermediates in an overall reaction.

True- because the species formed are unstable ions.

How are partial charges generated?

By the presence of a highly electronegative species in the molecule resulting in the polarization of a bond.

True or False: Alkenes (unsaturated compound, double-bond) are more reactive than alkanes (saturated compound, single-bond).

True- the C=C is a region of high electron density that is susceptible to electrophilic attack.

What is an electrophilic addition reaction?

An alkene and an electrophile

What is the general molecular formula for an electrophilic addition of a halogen?

C_nH_2nX₂

What is the general molecular formula for an electrophilic addition of a hydrogen halide?

C_nH_2n+1X₂

What is the general molecular formula for an electrophilic addition of water and what is its special case?

C_nH_2n+1OH

The alkene must be added to an acidic solution, resulting in a hydration reaction.

Hydration reaction

Where a molecule gains an H₂O molecule. This process involves adding a hydroxyl group (OH^-) and a hydrogen atom (H⁺) to different carbon atoms in a molecule.

Note: occurs during the electrophilic addition of water by adding the alkene in the acidic solution.