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(2) Main Group Chemistry > Group 13 > Flashcards

Group 13 Flashcards

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1
Q

What is the outer orbital configuration?

A

ns2np1

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2
Q

What is the inert pair effect?

A

The increasing stability of oxidation states that are 2 less than the maximum oxidation state of the group

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3
Q

What is passivated?

A

This is making a metal unreactive by altering the surface layer and coating the surface with a thin inert layer

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4
Q

What is the max oxidation state?

A

+3

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5
Q

Which oxidation state becomes more important down the group?

A

+1 oxidation state becomes more important down the group
The max oxidation state is +3
This shows the inert pair effect

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6
Q

Why are there no ionic compounds containing B3+?

A

The 1st 3 ionisation energies of B are high so there are no ionic compounds containing it
Instead it will form covalent compounds

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7
Q

Why are group 13 elements Lewis acids?

A

They have a low energy empty p orbital that can accept a pair of electrons

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8
Q

How do they overcome their lack of valence electrons?

A

Many compounds form 3 centre, 2 electron bonds

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9
Q

What is boron best described as?

A

Non metal whereas other G13 are metal

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10
Q

What is boron extracted from?

A

Deposits of borax

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11
Q

What is Al extracted from?

A

Extracted from the ore bauxite

1) Bauer haber process
2) hall herout process

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12
Q

Why can Al be used in power lines?

A

It is passivated so it is resistance to corrosion

- oxide acts as a physical barrier

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13
Q

Describe the trend in metallic character down the group

A

Metallic character increases going down the group- reflected by Li and Ti dissolving in acid

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14
Q

What causes anomalies to I.E. Trend?

A

The d and f block contraction which is characterised by the poor shielding effects of the 3d and 4f orbitals

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15
Q

What do G13 form when they burn in air?

A

Oxides

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16
Q

What is the general formula of a compound ?

A

MX3

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17
Q

Why do they display Lewis acid behaviour ?

A

They have 6 valence electrons and so can accept a pair of electrons

18
Q

Why do they have a lack of valency?

A

They only have 6 valence electrons

19
Q

What is the bond in B2H6?

A

3 centre, 2 electron bonding as B is electron deficient

20
Q

How is B2H6 synthesised?

Diborane

A

4BF3 + 3NaBF4 -> 3NaBF4 + B2H6

21
Q

Explain why B2H6 forms 3 centre 2 electron bonds

A

-Each B has 3 outer electrons and 4sp3 orbitals
-There are 12 electrons in total from 2B and 6H for bonding
-After the 4 terminal BH bonds have been made there are only 4 electrons left for last 2 hydrogen which require bridging bonds (2 bonds each)
This means 3 centre 2 electron bonds are formed (each contain 2 electrons)

22
Q

What hybridisation does boron have?

A

Boron is tetrahedral and so has sp3 hybridisation

23
Q

Use MO theory to describe the 3 centre 2 electron bonding in diborane

A 
There are 3 atomic orbitals involved in each 3 centre, 2 electron bond 
- an sp3 orbital from each B
- a 1s orbital from H
- This forms 3 molecular orbitals 
( bonding, antibonding and non bonding)
24
Q

How is the bonding, antibonding and non bonding MO in the 3 centre, 2 electron bonds formed in diborane?

A

Bonding MO- all AO are in the same phase

Antibonding MO- the 2 B AOs are the same phase, H AO is a different phase

Non bonding MO- the 2 B AOs are different phases and so H AO has the wrong shape

25
Q

What is a halide?

A

A binary compound of a halogen with another element or group

26
Q

Describe the structure and hybridisation of boron halides

A

Trigonal planar structures with sp2 hybridisation

27
Q

Why do B-X bonds in BX3 have double bond character?

A

Boron has a formally vacant p orbital which the halogens can donate their lone pairs into
This means each B-X has a little double bond character and changes the charge distribution

28
Q

What is the trend in acidity of boron halides?

A

BF3 < BCl3 < BBr3

Opposite to electronegativity

29
Q

What would be the expected trend in boron halide acidity and what is the actual trend caused by?

A

On basis of electronegativity you would expect the opposite (electronic effects making the most positive B atoms)
However this is not true, it is due to pi bonding contribution

30
Q

How does pi bonding contribution affect boron halide acidity?

A

Halogens have lone pairs they can donate to B empty p orbital
- B 2p and F2p are similar sizes so good overlap & more electron density can be donated
As halogens get bigger, the orbital size is less similar, poorer overlap and so less electron density is donated= more positive B atoms
This is explains trend in acidity

31
Q

What happens to structure and hybridisation when a BX3 reacts with a Lewis base?

A

Trigonal planar to tetrahedral

Sp2 to sp3

32
Q

What ppi-ppi bonding?

A

In BX3, X can donate electron density from filled p orbitals to empty p orbitals on boron
This creates ppi-ppi bonding

33
Q

Why is ppi bonding strongest in BF3 and weakest in BI3?

A

Pi bonding is distance sensitive and so ppi-ppi bonding decreases in strength as the size of the halogen increases

34
Q

Why does ppi-ppi bonding only occur with sp2 hybridised boron and not sp3?

A

There is no available orbital of the correct symmetry to interact with the halogen p orbital in sp3

35
Q

Why do larger halogens form adducts more readily?

A

Ppi-ppi bonding is weaker in larger halogens

This bonding is less when the tetrahedral Lewis acid- Lewis base adduct forms

36
Q

Why do boron halides not dimerise?

A
  • this is because BX3 is much more stable than its dimer because :
  • the presence of ppi-ppi bonding would be lost in the B2X6 dimer
  • there would be a greater steric bulk due to increase in coordination number
37
Q

Why do BX3 has shorter bond lengths than expected?

A
  • ppi-ppi bonding
  • the large electronegativity difference between B and X leads to ionic character in the bond
  • There are only 6 valence electrons and so less electron repulsions
38
Q

Describe the bridging bonding in Al2Br6

A

There are 3 centre, 4 electron interactions

Each bridging bromide donates all 3 of its valence electrons, 1 in a normal covalent bond and the other 2 in a dative covalent bond

39
Q

What drives the formation of the Al2X6 dimer?

A

This is driven by the high Lewis acidity of the aluminium centre (it wants to accept a pair of electrons from the bromine in the form a dative covalent bond)

40
Q

Why can Al form Al2X6 dimers with halogens whereas boron can only form halides BX3

A

Aluminium is larger than boron and so steric strain is less

(2) Main Group Chemistry (11 decks)

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