11. FGC 2 - Nucleophiles, Electrophiles and Reactivity

What are molecules coated with?

- Coated in a layer of electrons which occupy bonding and maybe non-bonding orbitals.

- As a result, the surface of each molecule is negatively charged, and therefore molecules repel each other.

What causes a reaction to occur?

- Reactions can only occur if a pair of molecules have enough energy to overcome this superficial repulsion.

What is the activation energy?

- The minimum amount of energy required to start a reaction.

what brings molecules together?

- Charge attraction or orbital overlap bring molecules together.

What is a common cause of organic reactions?

- A common cause is the attraction between a charged reagent (a cation or anion) and an organic compound that possess a dipole.

In general, how do molecules react?

- Molecules repel each other, and need to overcome a barrier with a minimum amount of activation energy in order to react.

What do most organic reactions involve interactions between?

- Involve interactions between full and empty orbitals.

- Many but not all, also involve charge interactions, which help overcome electronic repulsion.

What do some ionic reactions involve?

- Involve nothing but charge attraction.

What is a reaction essentially?

- A reaction is essentially the movement of electrons from one molecule to another (a pair of molecules must find themselves close together first), i.e. the mechanism of the reaction.

What is the 'mechanism of the reaction'?

- The detailed description of the pathway the electrons take.

What is an electrophile?

- The molecule that accepts the electrons (electron lover).

- The partially positively charged atom/functional group will accept and want these electrons.

What is a nucleophile?

- The molecule that donates the electrons.

What does a curly arrow represent?

- Movement of an electron pair.

Do you need orbital overlap for a reaction to occur?

- Yes, orbital overlap is essential for a reaction to occur.

- If empty orbitals doesn't line up between the reacting species, then the reaction will not occur.

How would you identify a nucleophile?

- Nucleophiles are either negatively charged or neutral species with a pair of electrons in a high-energy orbital.

Thee most common type of nucleophile has what?

- Has a non-bonding lone pair of electrons. (e.g. Nitrogen)

Are anions good nucleophiles?

- Yes, as they have a lone pair of electrons, e.g. NaOH (OH-).

- The anionic centre is usually O,S or a halogen, each of which can have several identical lone pairs.

How can molecules still be nucleophilic without non-bonding lone pairs?

- They can have the next highest set of orbitals instead, which are bonding pi orbitals, especially C=C, since they are higher in energy than sigma bonds.

How would you identify an electrophile?

- Electrophiles are neutral or positively charged species with an empty atomic orbital

• or a low-energy anti-bonding orbital that can easily accept electrons.

What are the most important functional group in organic chemistry?

- Carbonyl groups

In most organic electrophiles the LUMOs are associated with what? owest Unoccupied Molecular Orbital.

- They are low-energy anti bonding orbitals associated with electronegative atoms.

What can the anti-bonding orbitals either be?

- Either pi orbitals or sigma orbitals - in other words, molecules which make good electrophiles might have a double or a single bond to an electronegative atom such as O,N,Cl or Br. It's important that an electronegative atom is involved in order to lower the energy of the orbital.

Why is carbon versatile?

- It is electrophilic when bonded to a more electronegative atom.

- It is nucleophilic when bonded to a more nucleophilic element.

What does a curly arrow with a single arrow head represent?

- A single electron movement.

Whenever you move electrons, what effect does this have on the overall charge?

- Overall charge is always conserved in a reaction.

What is an SN1 reaction?

nucelophillic susbution?

- Nucleophilic substitution

- First order process (i.e. one molecule involved in the rate determining step).

What is an SN2 reaction?

nucelophillic sub

- Nucleophilic substitution

- 2nd order process (i.e. 2 molecules involved in the rate determining step).

Do nucleophiles or electrophiles attack each other?

- Nucleophiles attack electrophiles; we start off from high electron density and move towards low electron density.

Describe an SN1 mechanism.

2

how mnay steps?

molcules

1. Two-step

2. Unimolecular

Describe an SN2 reaction.

1. One-step

2. Involves 2 molecules

What does the rate of an SN2 reaction depend on?

- The nucleophile

- The carbon skeleton

- The leaving group

- Along with the usual factors of temperature and solvent.

What does the rate of an SN1 reaction depend on?

- The carbon skeleton

- The leaving group

- Along with the usual factors of temperature and solvent.

BUT NOT the nucleophile!!!

Why are tertiary groups not good for SN2 reactions?

- The nucleophile is sterically hindered.

- I.e. The groups get in the way, therefore it's just a physical repulsion and the nucleophile thus is not able to get to the carbon for the reaction.

How does stereochemistry relate to SN1 and SN2 reactions?

- SN1 reactions you loose stereo control and obtain a racemic mixture if you start with a chiral centre.

- SN2 reactions you invert the stereochemistry.

How does solvent effects relate to SN1 and SN2 reactions?

- SN1 reactions generally performed in polar protic solvents.

- SN2 reactions are generally performed in aprotic less polar solvents.

How do nucleophiles relate to SN1 and SN2 reactions?

- SN1 reactions the nucleophile isn't important

- SN2 reactions a good nucleophile is essential

2. Explain, briefly, the experimental evidence for and SN2 reactions

• RATE OF REACTION is dependent on the CONCENTRATION of the Haloalkane and the Nucleophile

• RATE OF REACTION with a given Nucleophile DECREASES with INCREASING bulk of the Haloalkane

• CHIRALITY of the substituted product is INVERTED . (Walden 1896)

Explain, briefly, the experimental evidence for SN1

RATE OF REACTION depends only on the CONCENTRATION of the Haloalkane

• RATE OF REACTION is favoured by the BULKINESS of the Alkyl substituent

• - SUBSTITUTION of a chiral alkyl halide results in a racemic mixture of products