Class 12 Chemistry (India)
- Identifying nucleophilic and electrophilic centers
- SN1 mechanism: Kinetics and substrates
- Sn1 mechanism: stereochemistry
- Carbocation stability: Recap
- Effect of substrate on the rate of an SN1 reaction- Part 1
- Effect of substrate on the rate of SN1 reaction-Part 2
- Identifying the major product- Carbocation rearrangement
- Effect of substrate on SN1 reactions (BASIC)
- Effect of substrate on SN1 reactions (INTERMEDIATE)
- Factors affecting SN1 reaction: leaving group and solvent effects.
- Sn2 mechanism: kinetics and substrate
- Factors affecting SN2 reactions: substrate effect.
- Effect of substrate of SN2 reactions
- Factors affecting SN2 reactions: leaving group- Part 1
- Factors affecting SN2 reactions: leaving group-Part 2
- Factors affecting SN2 reactions: Leaving group- Part 3
- Factors affecting leaving group ability - I
- Effect of leaving group on SN2 reactions - II
- Factors affecting SN2 reactions: Strength of a nucleophile
- Strength of a nucleophile in SN2 reactions
- Practice questions on SN2 reactions
- Sn1 vs Sn2: Summary
- Kinetics of SN1 vs SN2 reactions
- Comparing E2, E1, Sn2, Sn1 reactions
- E2 E1 Sn2 Sn1 reactions example 2
- E2 E1 Sn2 Sn1 reactions example 3
The definition of nucleophiles and electrophiles. Identifying nucleophilic and electrophilic centers in a molecule. Created by Jay.
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- So I'm not understanding why there is a molecule with a partially positive charge (carbon) and partially negative charge (Chlorine), and it is said to be an electrophile because of the partially positive charge on Carbon. Why couldn't it be a nucleophile because of the partially negative charge on Chlorine?(9 votes)
- Given that the other reactant, ethoxide, is a much stronger nucleophile than chlorine would be in this compound, the reaction will favor the ethoxide as the nucleophile. Also, for chlorine to act as a nucleophile it would have to break its bond to carbon as it only needs one more electron, thus one bond, to form a stable octet.(6 votes)
- So does every compound have a nucleophilic and an electrophilic centre simultaneously?(11 votes)
- you said that the carbon atom surrounded by three methyl groups will be an electrophile.....shouldnt it be a nucleophile cause it will have one free electron...?O.o(8 votes)
- he draw the carbon surrounded by three methyl groups as a tertiary carbocation, so it has a positive charge and no free electron, so it would like to have one electron more, then its called an electrophile.(5 votes)
- Can someone please explain how NH3(that's supposed to be ammonia) is a nucleophile?(2 votes)
- A nucleophile is a species that is strongly attracted to a region of positive charge on a carbon atom in another molecule.
In NH₃, N is more electronegative than H, so the N atom has a partial negative (δ⁻) charge. It also has a lone pair of electrons.
In a molecule like CH₃CH₂-Br, Br is more electronegative than C, so C-1 has a partial positive (δ⁺) charge.
The lone pair on the N can attack the δ⁺ carbon on CH₃CH₂Br:
H₃N: + Br-CH₂CH₃ → H₃N⁺- CH₂CH₃ + :Br⁻
When NH₃ behaves in this way, it is acting as a nucleophile.(7 votes)
- wait....im so confused...so oxygen is nuceophilic but Chlorine is electrophile? please explain this. im not understanding how to differentiate between the two completely(3 votes)
- Your right that oxygen is a nucleophile, but you are very wrong about chlorine being an electrophile. In the case in the video, the adjacent carbon is considered to be the electrophile not the chlorine. The pull of electrons in the polar bond between the carbon and chlorine cause a charge distribution between the two atoms based on electronegative effects. The carbon gains a partial positive charge and the chlorine gains a partial negative charge. The positively charged carbon in this case would be the electrophile. Chlorine is often a nucleophile, as you will find in halide reactions within organic chemistry. I would suggest going over electronegativites and the 5 schwarts guidelines.(2 votes)
- How big does the difference in electronegativity between two atoms have to be for us to treat it like an ionic bond, like Jay does at2:00?(2 votes)
- Generally around 1.7, that is when the bond has 50% ionic character.
Different texts may use slightly different numbers(3 votes)
- could we have made 4 different structures in the last section..
why only 3?...(2 votes)
- What does pKa and pKb stand for?(1 vote)
- pKa = -log(Ka) where Ka is the acid dissociation constant.
pKb = -log(Kb) where Kb is the base dissociation constant.
Ka's and Kb's are often very big or very small numbers, so by using (negative) logarithms, the numbers are much more manageable.(5 votes)
- Is cl- a nucleophile or electrophile?(2 votes)
- A nucleophile because of its negative charge. Electrophiles often have positive charges. Another way to think about it is that because nucleophilicity and basicity are closely related, the chloride ion is a nucleophile because it is also the conjugate base of hydrochloric acid. Hope that helps.(2 votes)
- I know this has been asked many times but I am still a bit confused. Is the reason why the chlorine is a bad nucleophile(on the alkyl halide example) because since it is a very weak base, it is typically a leaving group instead. But it is still possible that Cl does act as a nucleophile depending on the conditions, like ketone can act as a nucleophile/electrophile depending on the conditions, right?(2 votes)
- So first we're not really being shown chlorine as a nucleophile here. Instead we're being shown that it can act as a good leaving group and make the alkyl halide a good electrophile. Once the chlorine has left, it will becomes chloride and can act as a nucleophile.
We decide how good a nucleophile is based on how fast it can react as a nucleophile compared to other nucleophiles. With this in mind, I wouldn't classify chloride as a weak nucleophile, rather a moderate one. Chloride is a stronger nucleophile than fluoride but a weaker one than iodide (at least in protic solvents).
The main reason for halogen's nucleophilicity and their differences is their polarizability. Polarizability in this context being how loosely outer electrons are held onto by the nucleus. Fluoride is not very polarizable since it has smaller electron shells tightly held onto by the nucleus. And iodide is very polarizable since it has very large electron shells not so tightly held onto by the nucleus. A more polarizable ion is able to share it electrons more easily in a nucleophilic attack.
So chloride is a moderately polarizable ion, able to hold into its outer electrons less so than fluoride but more so than iodide. And this is the main reason and differences in halogen's nucleophilicity.
Hope that helps.(2 votes)
- [Instructor] Nucleophiles and electrophiles are extremely important in organic chemistry mechanisms. So, first let's look at a nucleophile. The word nucleophile means nucleus-loving and since the nucleus is positively charged you can think about a nucleophile as being negatively charged because opposite charges attract. So a nucleophile could have a full negative charge which would be attracted to the positive charge of a nucleus or it could have a partial negative charge or you could just think about a nucleophile as having a region of high electron density. So let's look at some examples of nucleophiles. First let's start with the ethoxide anion here and the ethoxide anion has an oxygen with a full negative charge, so obviously that is a nucleophile and the oxygen is the nucleophilic center of ethoxide. Next let's look at ethanol. Well ethanol doesn't have a full negative charge but we know that oxygen is more electronegative than hydrogen so oxygen is going to pull the electrons in this bond closer to itself, giving it a partial negative charge, so this oxygen is the nucleophilic center of ethanol. Now the ethoxide anion is going to be a better nucleophile than ethanol because it has a full negative formal charge on the oxygen as opposed to only a partial negative. Next lets look at methyllithium. Let's think about the electronegativity difference between carbon and lithium. Carbon is more electronegative than lithium so the two electrons in this bond are pulled closer to the carbon, giving the carbon a partial negative charge, and so the carbon is the nucleophilic center of methyllithium. Since lithium is losing some electron density we could draw a partial positive charge here on lithium and here I've drawn it as a covalent bond but really you could also show it as being an ionic bond. So the difference in electronegativity is so great that I could show both of those electrons being on this carbon, so let me go ahead and put in the hydrogens here. Since carbon is more electronegative than lithium I could take these two electrons in magenta, and I could put them both on the carbon, which would give the carbon a negative one formal charge so this carbon with a negative one formal charge would be the nucleophilic center. I took an electron away from lithium giving it a plus one formal charge here, so here I've represented it as an ionic bond, here a little more covalent character, but this picture is useful because this is called a carbanion, let me write this in here, so a carbanion which just means a negative charge on a carbon and carbanions are excellent nucleophiles. Finally, let's look at cyclohexene and cyclohexene we know has a pi bond. So let me show the pi bond here, and pi bonds are regions of high electron density so this pi bond can act like a nucleophile in an organic chemistry mechanism. Now let's look at electrophiles. So an electrophile is electron-loving and since electrons are negatively charged we're gonna think about an electrophile as having a region of low electron density so it could have a full positive charge on it because a positive charge would be attracted to electrons, or you could be talking about a partial positive charge. So first let's look at this compound. We know that chlorine is more electronegative than carbon. So chlorine is going to withdraw some electron density and if chlorine is withdrawing electron density away from this carbon, this carbon is partially positive. So this carbon is the electrophilic center of this compound. Next let's look at acetone. So oxygen is more electronegative than carbon so oxygen is going to withdraw some electron density away from this carbon here and this carbon would be partially positive, so this carbon is the electrophilic portion of this compound. Next let's look at a carbocation where there's a full positive charge on this carbon so this carbon has only three bonds to it which gives it a full positive charge. Obviously a full positive charge is going to love electrons. Opposite charges attract, so this carbon is the electrophilic portion of this ion. And finally let's look at this compound, right. We know that oxygen is more electronegative than carbon so oxygen withdraws some electron density away from this carbon and we could even draw a resonance structure here, so let me take these pi electrons and move them out onto the oxygen, so let's draw a resonance structure so I put in my double bond. Now if I'm showing those pi electrons moving off onto the oxygen I would need three lone pairs of electrons on that top oxygen giving it a negative one formal charge. I took a bond away from this carbon in magenta which is this carbon which gives it a plus one formal charge, so that's one of the possible resonance structures that you can draw and of course we know the carbon in magenta is an electrophilic center, but I could draw another resonance structure so let me go ahead and do that, put in my brackets over here. I could take these pi electrons, I'll show it on this one actually, these pi electrons and move them over to here, so let's draw the resulting resonance structure. So I'd have a double bond here now an oxygen with a negative one formal charge, so let me put that in here, draw in the hydrogen, put in my brackets, and I removed a bond, we took a bond away from, let me use blue for this, from this carbon, so this carbon now has a plus one formal charge, so the carbon in blue is this carbon over here, so let me draw in a plus one formal charge, so that is also electrophilic, right, a full positive charge is going to be attracted to a negative charge, so this compound actually has, this compound actually has two electrophilic centers, so this carbon here and also this carbon.