Lecture 5 - electronegativity, pKa and the inductive effect. Flashcards
Define electronegativity
a chemical property that describes the tendency of an atom to attract a shared pair of electrons towards itself.
What effects electronegativity?
- nuclear charge
- shells and shielding
- number and location of electrons in the atomic shells
How does electronegativity increase across periodic table?
From bottom left to top right.
Across periods it increases due to an increase in nuclear charge.
Down groups it decreases due to shielding.
Electronegativity of fluorine
3.98
Electronegativity of hydrogen
2.2
Electropositivity
the measure of elements ability to donate electrons (the opposite of electronegativity)
Define the inductive effect.
The shifting of electrons in a sigma bond in response to the electronegativity of the bonding atoms.
Describe the inductive effect in HCl.
In HCl, chlorine is more electronegative than hydrogen. Therefore, the electron cloud is pulled towards the chlorine atom.
There is a shift in the electron density- it shifts towards the chlorine. Chlorine becomes partially negatively charged and hydrogen partially positively charged.
Metals have high or low electronegativity?
low
We say that metals…
inductively donate electrons
Non metals have high or low electronegativity values?
high
We can say that nonmetals….
inductively withdraw electrons
How should the inductive effect be drawn?
- Arrow on bond indicates which way the electron density is going.
- Electrons are displaced in the direction of the arrow
What is a Bronsted lowry acid?
A substance which donates a hydrogen ion, H+
What is a Bronsted lowry base?
A substance that accepts hydrogen ions
Give the equation for when HCl dissolves in water. Explain which species is the acid and which is the base, which is the conjugate acid and which is the conjugate base?
When HCl (g) dissolves in water the HCl acts as an acid and donates a proton. The water acts as a base and accepts the proton. A hydronium ion, H3O+ and a chloride ion Cl- is formed.
HCl + H2O –> Cl- + H3O+
The chloride ion is called the conjugate base of the acid and the hydronium ion is the conjugate acid of the base.
Water is both a …….. an an ……….
base
acid
Give the reaction of water with ammonia.
In a reaction with ammonia, NH3, water is an acid that donates a proton to produce ammonium ion NH4+ and a hydroxide ion, OH-
NH3 + H2O –> NH4+ + OH-
What is a strong acid?
completely dissociate in water-donate all of their H+ ions.
What is a weak acid?
An acid which only partially dissociates in water.
The strength of an acid in water solution is described using, what?
Ka- the acid dissociation constant.
Give the equation for Ka
HA ⇌ H^(+ )+ A^-
Ka= ([ H^(+ ) ][A^-])/([HA])
How does the addition of chlorine atoms to carboxylic acid molecules reduce the pKa of these acids (make them more acidic)?
This is related to the inductive effect.
- stronger acid = weaker/more stable conjugate base
- The addition of chlorine introduces a C-Cl bond, which is polar. Chlorine is more electronegative than carbon and has a stronger electrostatic attraction for the electrons in the bond. The electron density shifts towards the chlorine.
- The negative charge on the carboxylate ion of the acids are is stabilised by the electron withdrawing C-Cl bond (the partially positive carbon). So, the conjugate base is now stabilised due to the inductive effect. The more stable the conjugate base, the stronger the acid
The pKa will then drop. The acid becomes stronger when there are more Cl-C groups added
Explain resonance using acetate ions. Draw the two resonance forms
The Lewis structure below appears to tell us that one oxygen is more negative than another and therefore one oxygen should be more reactive than another.
In reality, both oxygens are equally reactive with hydrogen….
There are two possible structures that may be produced because both oxygens are equally reactive. This is because there are two resonance forms of acetate ion/ ethanoic acid.
We can draw a hybrid of the two resonance forms to show the true electron densities in acetate ion…
Draw the two ways you can represent benzene. Why do we sometimes show the single/double bonds?
to show the possibility of resonance forms.
Describe resonance in benzene. Draw it.
Resonance is not about moving atoms around; it is about moving electrons around the benzene ring. When the electrons are moved, the placement of the bond shifts. When you move the electrons, you make a bond. But, when you make a bond, you break a bond. The movement of each of these bonds happens at the same time
Explain resonance in carboxylate anions. Draw it.
- A carboxylate anion is formed when carboxylic acids dissociate in water and release H+ ions.
- The oxygen in a carboxylate ion has a negative charge. However, oxygen is not happy to have a full negative charge. So, what it does is it tries to spread the electron density into the C-O bond, making a new double bond as a result. But, because this bond has been made, the C=O bond will then break (if you make a bond, you break a bond). The oxygen that was involved in this C=O bond will now have a negative charge. It is almost like it has flipped.
- The negative charge of the oxygen is spread out over the whole system to make it happier
Describe resonance in phenol/ phenolate ions
Phenol is a weak acid. It can dissociate to form a proton and a cation- a phenolate ion. But the negatively charged oxygen in the phenolate ion doesn’t want that negative charge. So, the negative charge is delocalised into the pi ring system. By doing that a bond is made in the ring with a carbon, but the carbon would then have 5 bonds. So, it breaks a bond. The carbon then has a negative charge. So it then donates an electron to another carbon and the process continues. The negative charge is delocalised around the aromatic ring, being spread out and stabilised.
Substituted phenol molecules can show the mesomeric effect. Describe the mesomeric effect with the strong M+ donator, amine NH2.
When bonded to a benzene ring, the nitrogen in NH2 has an orbital hybridisation of Sp2, with bond angle 120. (it is the exception to the rule we can use to work out hybridisation…number of atoms bonded+ lone pairs). The lone pair in this nitrogen is therefore in the unhybridized p orbital and the three sp2 orbitals are involved in bonds to the two hydrogen and to a carbon in the benzene ring. The lone pair of electrons in the p orbital overlaps with the pi system. The electron density is pushed into the pi system, increasing the electron density of the pi system. This is the positive mesomeric effect.
Why does nitrophenol have a lower pKa than phenol?
- The positively charged nitrogen in this nitro group is Sp2 hybridised. It’s unhybridized p orbital does not contain a lone pair of electrons- it is deficient in electron density. As a result, when the p orbital overlaps with the pi system, it removes electron density from the pi system
- The negatively charged oxygen ion in the nitrophenolate of nitrophenol delocalises electrons into the pi system- +M mesomeric effect. The base is now stabilised and has a reduced tendency to accept protons.
- It is able to delocalise it’s electron out onto the nitro group
- The system is now more stabilised than an ordinary phenolate.
- More stable base= stronger acid = lower pKa.
The mesomeric effect for amines- Why does phenylamine have a lower pKa than cyclohexylamine
- The nitrogen of phenylamine is sp2 hybridised. The lone pair of electrons is in an unhybridized p orbital and can overlap with the aromatic pi system. – it is a +M mesomeric effect.
- Because it has overlapped with the aromatic system, electron density is pushed into the pi system- this increases the electron density of the pi system but it reduced N’s ability to pick up a proton. It is a weaker base, the conjugate acid is stronger, so the pKa is lower.
- In contrast, in cyclohexylamine, the nitrogen is sp3 hybridized- it is not bonded to a pi system (so the rule about bonding atoms + electron pairs works- the exception was when p orbitals were adjacent to pi systems) - and there is a lone pair in one of the sp3 orbitals. This lone pair is not pushed into the pi system- there isn’t one- so it has a better ability to pick up protons and is a stronger base, conj acid is therefore weaker, it has a higher pKa.
