ION organometallics

Question 1

What is an organolithium and how do they react?

Answer 1

• contain a C-Li bond

- act as ionic species, they are very reactive

Question 2

Draw n-BuLi, s-BuLi and t-BuLi

Answer 2

n is straight alkyl chain, s 1st carbon is attached to two other carbons, t is like a pitchfork

Question 3

Which is the most reactive of the simple organolithiums?

Answer 3

t-BuLi

Question 4

Give some examples of donating solvents used to increase the reactivity of organolithiums. How do they work?

Answer 4

DME (1,2-Dimethoxyethane), THF (Tetrahydrofuran), Et2O (diethylether), TMEDA(tetramethylethylenediamine)
-They stabilising the Li cation by providing electron density and causing deaggregation ( usually have an N or O)

Question 5

What is the chelate effect and which solvents does it apply to?

Answer 5

• stabilisation of the Li cation by coordination with TWO lone pairs
• increases the reactivity of the organolithium by lower the aggregation state of the reagent
• works with DME and even better with TMEDA

Question 6

Draw HMPA and state why it is good at stabilising lithium cations

Answer 6

• P=O bond is v. polar w/ -ve charge on oxygen which can coordinate with the Li+
• N lone pairs can stabilise the +ve P

Question 7

simple alkyl and aryl organolithiums are v. reactive with _______________

Answer 7

H2O, O2 and CO2

Question 8

At what temperature do organolithium reactions usually take place and why?

Answer 8

• 78C so that there is no reaction with the ether solvent

- At high temps the RLi will react with the solvent especially THF

Question 9

How are organolithiums made? What drives the reaction?

Answer 9

Reacting an alkyl or aryl halide with lithium metal. This doesn’t work with fluorine.

Question 10

What are the two methods of synthesising functional organolithiums?

Answer 10

1. Deprotonation

2. Transmetallation and halogen-metal exchange

Question 11

Describe the process of deprotonation

Answer 11

A highly reactive organolithium is used to take a proton off another organic molecule. For example R-Li reacts with R’-H to make R-H and R’-Li. R-Li is usually n-BuLi so n-BuLi + R’-H —> BuH (butane gas) + R’-Li

Question 12

Describe the process of transmetallation and halogen-metal exchange

Answer 12

An alkyl or aryl bromide or iodide is reacted with an organolithium, this is an equilibrium reaction which favours the more stable organolithium
i.e. R’-Br + BuLi —> R’-Li + BuBr

Question 13

What is the formula of Grignard reagents?

Answer 13

RMgX X= Cl, Br, I

Question 14

Why are Grignard reagents less reactive than organolithiums?

Answer 14

The C-Mg bond is less polar

Question 15

How are Grignard reagents prepared?

Answer 15

By reacting Mg with the corresponding alkyl or aryl chloride, bromide or iodide

Question 16

Why is water excluded from reactions involving organometallics?

Answer 16

They react rapidly with water

Question 17

What are primary alcohols oxidised to?

Answer 17

Aldehydes and then carboxylic acids (hard to isolate them at an aldehyde) using CrO3

Question 18

What are secondary alcohols oxidised to?

Answer 18

ketones using CrO3

Question 19

What are tertiary alcohol oxidised to?

Answer 19

Tertiary alcohols are generally inert to further oxidation using CrO3

Question 20

What is retrosynthesis?

Answer 20

working backwards towards a starting material

Question 21

What is a synthon?

Answer 21

A generalised fragment, usually an ion produced by disconnection

Question 22

A good leaving group is ________________________

Answer 22

The conjugate base of a strong acid

Question 23

Which halogen would make the best leaving group and why?

Answer 23

Iodine because the C-I bond is the weakest of the halogens, bc. of lack of ionic contribution