can you please help me...
im really struggling to know which mechanism to use for with thing in organic chemistry.

how do i know when to use free radical, nucleophillic and eletrophillic? what reactants (substances) do i need to be present in order to use each one of these?

for free radical is it an alkane and halogen?
for electrophillic is it an alkene and hydrogen halide?
and i have no idea what it is for nucleophillic

thank you

Reply 50

kiiten

Original post by Squishy•

free energy and entropy

Isn't that A2 chem?

Reply 51

kiiten

Quick ques:In nucleophilic substitution reactions is the nucleophile dissolved in ethanol or the haloalkane?

Reply 52

richpanda

Original post by kiiten

Quick ques:In nucleophilic substitution reactions is the nucleophile dissolved in ethanol or the haloalkane?

Ethanol, not entirely sure what you mean. Ethanol is a good solvent for chemicals such as Ammonia and Cyanide.

Reply 53

kiiten

Original post by richpanda

Ethanol, not entirely sure what you mean. Ethanol is a good solvent for chemicals such as Ammonia and Cyanide.

I have seen that some books say the nucleophile is dissolved in ethanol but others say the haloalkane is dissolved in ethanol in a nucleophilic substitution. I was wondering which one it is or is it both?

Reply 54

richpanda

Original post by kiiten

Both.

Reply 55

Squishy•

Original post by libbyx

Basically trying to understand the mechanism is the best possible thing and understand these alongside understanding what a electrophile and a Nucleophile is.

So for example Alkene and a hydrogen bromide, the double bond is attacked by the slightly positive H atom in HBr and that causes the H-Br to fall onto Br. The way the double bond is attacked the H atom is showing that it's electrophillc addition because the double bond is donating a pair of electrons to the Hydrogen atom. As the original molecule (Alkene) is donating a pair of electrons to a positive atom, then it is electrophillc addition. :Br- is formed and attacks the cabocation (positive charge on carbon as it only has three bonds).
An electrophilic addition reaction is an addition reaction which happens because what we think of as the "important" molecule is attacked by an electrophile. The "important" molecule has a region of high electron density which is attacked by something carrying some degree of positive charge.
An addition reaction is a reaction in which two molecules join together to make a bigger one. Nothing is lost in the process. All the atoms in the original molecules are found in the bigger one.

A nucleophillic substitution reaction is where a Nucleophile (molecule that is negative or slightly negative/lone pair donor) attacks slightly positive atom in a polar molecule, removing the electronegative atom and replacing it. So for example the Nucleophile :Nu- attacks the slightly positive carbon in CH3CH2Br (second carbon is slightly positive due to electro negativity of Br) so then as the :Nu- bonds to carbon the carbon pushes off the Br and the Br is replaced by Nucleophile. So nucleophillic substitution reactions occurs when a Nucleophile substitutes an electronegative atom (halogen).

Free radicals are atoms or groups of atoms which have a single unpaired electron. A free radical substitution reaction is one involving these radicals.
A free radical can occur if a diatomic molecule of a halogen is split up with U.V energy. This produces 2 free radical halogens(2Cl•).
Free radical substitution involves breaking a hydrogen bond.
The chain initiation is:
Cl2 -> 2Cl•
This initiates the chain propagation reaction which is:
E.g: Cl•+CH4 -> CH3• + HCl (C-H bond broke)
CH3• + Cl2 -> CH3Cl + Cl• (radical produced in these as these cause a chain reaction)
Termination:
CH3• + Cl• -> CH3Cl
And so on.

So you were right in the electrophillc and free radical but to know when they occurs you have to look at the presence of an electrophile and Free radical. If you were told to write the mechanism for a reaction, if the main molecule attacks a positive part of the other molecule then it's electrophillc addition. If the main molecule is attacked by a negative molecule and an atom is replaced then it's nucleophillic substitution. Free radical is when a halogen diatomic molecule is split into two free radicals.

If you still don't understand something just mention it again and I'll narrow my respond more.

Reply 56

Squishy•

Original post by kiiten

When there is a Nucleophile, dissolving it in water can cause the OH- Nucleophile from the water to act as a competitor so it can be dissolved in ethanol to prevent this. Unless of course they're using OH- Nucleophile and don't mind.
When elimination reactions occur, NaOH is dissolved in ethanol as the OH- acts as a base in that solution (loves protons) and attacks a proton (H+) instead so an elimination reaction occurs and not always a nucleophillic substitution.

(edited 8 years ago)

Reply 57

t.thompson22

Original post by libbyx

For Free Radical Substitution you will be asked to write the Initiation, Propagation I, Propagation II and Termination steps. You will be reacting any Alkane with a Halogen Free Radical atom, such as Cl• with the • representing an unpaired electron.

For Nucelophillc Substitution you're reacting a Halogenoalkane such as Bromoethane with another halogen atom such as Cl- to convert the reactant into Chloroethane.

Electrophillic Addition takes place between an electrophile such as HBr, or Br2 and an Alkene to convert the Alkenes carbon-carbon double bond into a Halogenoalkane, which has unsaturated carbon-carbon bonds only. This is because the Electrophile acts as an electron-pair acceptor from the double covalent bond in an alkene.