hybridisation i gjrowlan/intro/lecture4.pdf · pdf file hybridisation: sp2 orbitals...

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  • 1

    Hybridisation I • We want model that describes 'localised' bonds • i.e. a model that matches Lewis structures (line diagrams) • Use hybrid AO (HAO) • Combine AO of each atom BEFORE combining atoms!

    H C C H

    Ethyne • Combine carbon AO first • Only combine (hybridise) AO we need • Each carbon attached to 2 groups so hybridise 2 AO • Remember conservation of orbitals

    2s

    2px 2py 2pz

    2(sp) 2(sp)

    2py 2pz

    carbon AO carbon hybrid orbitals 2 x sp (HAO) + 2 x p (AO)

    combine

    en er

    gy

  • 2

    =– =

    + =

    2s 2px

    Hybridisation II

    • We form 2 new hybrid AO called sp orbital • sp has 1/2s character & 1/

    2p character • Due to direction of

    combined AO new HAO are at 180˚

    2s 2px 2py 2pz

    + + +

    2py 2pz2 x sp AOsame plane 180˚

    Carbon Atomic Orbitals

    Carbon with 2 Hybrid Orbitals

    2 x sp 2 x AO

    +

    =Combination of 2 x carbon AO

  • 3

    π (2py + 2py)

    CH HC σ (1s + sp)

    σ (sp + sp)

    C H+H

    1s 2py

    2 x sp

    Ethyne: hybrid orbitals

    • Now combine HAO • Remember each HAO has 1 electron • So each MO has 2 electrons • 2π MO are perpendicular to rest of molecule • REMEMBER each p & π orbital has 2 lobes but is only 1 orbital! • REMEMBER antibonding orbitals also formed (not shown!) • Molecule is linear (straight) • Just one of many models (just very useful!)

    H C C H σ π

    π

    π (2pz + 2pz)2pz

    C

  • 4

    Hybridisation: sp2 orbitals

    Ethene • Each carbon attached to 3 groups so hybridise 3 AO • Produces 3 HAO - the sp2 orbitals • sp2 orbitals have 1/3s character & 2/3p character • Due to direction of combined AO new HAO are at 120˚ & all in the

    same plane

    C C H

    H H

    H

    2s 2px 2py 2pz

    + + +

    3 x sp2 all in the plane

    2pz

    120˚

    Carbon Atomic Orbitals

    Carbon with 3 Hybrid Orbitals

    3 x sp2 1 x AO

  • 5

    C C H

    HH

    H

    117.8˚

    σ (1s + sp2) σ (sp2 + sp2)

    π (2pz + 2pz)12 AO 12 MO 6 bonding (shown) +

    6 antibonding (not shown)

    H

    H

    + C

    H

    H90˚ 2pz

    C

    sp2

    Ethene

    • Simply combine HAO to give σ MO • Combine AO to give π orbital at 90˚ to rest of

    molecule • Remember it is one orbital with 2 phases

    C C H

    H H

    π

    σ

  • 6

    MO theory for ethene

    Bonding

    Antibonding

    HOMO highest occupied molecular orbital

    π orbital

    LUMO lowest unoccupied molecular orbital

    π* orbital

  • 7

    Hybridisation: sp3 orbitals

    Methane • Each carbon attached to 4 groups so hybridise all AO • Produces 4 HAO - the sp3 orbitals • sp3 orbitals have 1/4s character & 2/3p character • Due to direction of combined AO new HAO are at 109.5˚ in a

    tetrahedral arrangement

    H C

    HH H

    C

    2s 2px 2py 2pz

    + + +

    4 x sp3 tetrahedral

    109.5˚

    Carbon Atomic Orbitals

    Carbon with 4 Hybrid Orbitals

    4 x sp3

  • 8

    + C

    H

    H H

    C

    H

    H H

    σ (1s + sp3)

    σ (sp3 + sp3)

    C

    H

    HH H

    4 identical MO in tetrahedron with σ symmetry (bonds)

    σ (1s + sp3)

    + 4 H

    s AO of hydrogen

    Alkanes

    4 sp3 orbitals form tetrahedron

    C

    H C C

    HH

    H H

    H

    C

    H

    H H

    C

    H

    HH

    H C

    HH H

  • 9

    Hybridisation works for all atoms

    1s1s

    2s

    2px 2py 2pz

    2(sp3) 2(sp3) 2(sp3) 2(sp3)

    nitrogen AO nitrogen hybrid orbitals

    combine

    en er

    gy

    N H

    H H N

    HH H

    Ammonia • Nitrogen attached to 4 groups (we count

    lone pair) • So it will be sp3 hybridised • Combine with 3 x 1s of H • Gives 3 x s bonds & lone pair in sp3 orbital

  • 10

    Hybrid orbitals Borane BH3

    • Boron attached to 3 groups so hybridise 3 AO • So boron sp2 and trigonal planar • Has empty p orbital (AO)

    H

    C H

    H H

    H

    B H

    H H

    H

    N H

    H H

    • Isoelectronic (same number of electrons) so have the same MO! • Only difference is the energy and hence size

    H B H

    H BH H

    H B = sp2

    empty 2p empty 2p

    σ (1s + 2(sp2))

  • 11

    MO works for all molecules

    • 34 AO give 17 new bonding MOs (shown) & 17 antibonding MOs • It is only a model - but a very useful one!

    O

    C C

    O CC

    H

    H H H H

    H

    C sp2

    σ (1s + 2sp2)

    σ (C2sp2 + C2sp2)

    π (2p + 2p)

    O sp3lone pair sp3

    C sp3 σ (1s + 2sp3)

    σ (1s + 2sp2)

    π (O2p + C2p)

    lone pair sp2

    σ (C2sp2 + O2sp3)

    σ (C2sp2 + O2sp2)

    O sp2

    O H

    O

  • 12

    Reactions & Lewis structures • Organic chemistry & reactions is about making & breaking bonds • Or moving electrons • Below is the reaction of an alkene with bromine

    + Br Br + Br Br

    + Br Br Br

    Br

    colourless red colourless red

    colourless red colourless

    • Could draw out all the orbitals each time to explain reaction • Too much like hard work! • Use the diagrams above - but what are they actually showing?

    H H = = H H H Hx=

    σ (1s+1s) containing 2electrons

    H H

  • 13

    ≡ H FFH

    C H H

    H H H C

    H

    H

    H

    ≡C + 4H

    H

    Lewis structures: electron bookkeeping How do you draw a Lewis structure?

    • Draw a dot for each valence electron on each atom (old group number) • Share electrons (form bonds) to get duplet (H) or octet (the rest) • Third row can have >8 electrons • Each bond (line) is TWO electrons

    H

    Be B C N O F

    He NeLi

    lithium = 1 valence electron

    carbon = 4 valence electrons

    oxygen = 6 valence electrons

    1st row

    2nd row

    group 1 2 3 4 5 6 7 8

    C C

    valence electrons

    HF

    Examples

    CH4

    + F

  • 14

    C C H H

    H H

    ≡ C C H

    HH

    H C + 4HC +

    C H

    H H O H ≡ H C

    H

    O

    H

    HC + + 4HO

    Examples of Lewis structures

    • Remember: Draw lone-pairs (unshared electrons) - they are important • More than one covalent bond may be required! • Atoms can have less than an octet (very reactive!)

    C + + 6HO H3C

    C CH3

    O ≡ O

    3

    F ≡B + 3 FB FF F B F F

    BF3

    acetone

    CH2CH2

    CH3OH