ION organometallics Flashcards
What is an organolithium and how do they react?
- contain a C-Li bond
- act as ionic species, they are very reactive
Draw n-BuLi, s-BuLi and t-BuLi
n is straight alkyl chain, s 1st carbon is attached to two other carbons, t is like a pitchfork
Which is the most reactive of the simple organolithiums?
t-BuLi
Give some examples of donating solvents used to increase the reactivity of organolithiums. How do they work?
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)
What is the chelate effect and which solvents does it apply to?
- 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
Draw HMPA and state why it is good at stabilising lithium cations
- 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
simple alkyl and aryl organolithiums are v. reactive with _______________
H2O, O2 and CO2
At what temperature do organolithium reactions usually take place and why?
- 78C so that there is no reaction with the ether solvent
- At high temps the RLi will react with the solvent especially THF
How are organolithiums made? What drives the reaction?
Reacting an alkyl or aryl halide with lithium metal. This doesn’t work with fluorine.
What are the two methods of synthesising functional organolithiums?
- Deprotonation
2. Transmetallation and halogen-metal exchange
Describe the process of deprotonation
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
Describe the process of transmetallation and halogen-metal exchange
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
What is the formula of Grignard reagents?
RMgX X= Cl, Br, I
Why are Grignard reagents less reactive than organolithiums?
The C-Mg bond is less polar
How are Grignard reagents prepared?
By reacting Mg with the corresponding alkyl or aryl chloride, bromide or iodide
Why is water excluded from reactions involving organometallics?
They react rapidly with water
What are primary alcohols oxidised to?
Aldehydes and then carboxylic acids (hard to isolate them at an aldehyde) using CrO3
What are secondary alcohols oxidised to?
ketones using CrO3
What are tertiary alcohol oxidised to?
Tertiary alcohols are generally inert to further oxidation using CrO3
What is retrosynthesis?
working backwards towards a starting material
What is a synthon?
A generalised fragment, usually an ion produced by disconnection
A good leaving group is ________________________
The conjugate base of a strong acid
Which halogen would make the best leaving group and why?
Iodine because the C-I bond is the weakest of the halogens, bc. of lack of ionic contribution
