Honors Chemistry Chap 13Molecular Structure

13.1 Electron DistributionConsider 2 models of molec

structure which account for their shape

1st model takes into account the repulsive forces of e- pairs

2nd model considers ways in which orbitals can overlap to form orbitals around more than 1 nucleus◦E-’s in these orbitals bind the atoms

together

13.1 Electron DistributionIt’s useful to use Lewis e- dot

diagrams to describe the shape of molecs. or polyatomic ions

Shaired Pairs – prs of e-’s involved in bonding

Unshared Pairs – prs. of e-’s not involved in bonding◦Lone pairs

13.2 Electron Pair RepulsionAlso called VESPR Theory

◦Valence Electron Shared Pair Repulsion Theory

13.2 Electron Pair RepulsionOne way to account for molec

shape is to look @ e- repulsion◦Ea bond & lone pair in outer level for

a charge cloud that repels other chg clouds – due to like charges Also due to Pauli Exclusion Principle – e-

of like spins may not occupy the same vole of space

Repulsion due to like spins is much greater than repulsion due to like charges

13.2 Electron Pair RepulsionRepulsion betw chg clouds

determine arrangement of orbitals & \ the shape of the molec

Electron prs spead as far apart as possible to minimize repulsive forces.

If there are 2 e- prs, they will be on opp sides of the nucleus◦Linear (180o apart)

13.2 Electron Pair Repulsion3 e- prs – axes of chg clouds will

be 120o apart◦Trigonal Planar E- prs lie in the same plane as the

nucleus4 prs – axes of chg clouds will be

as far apart as possible – 109.5o

◦Tetrahedral Will not lie in the same plane 4 faces; ea is an equilateral triangle w/

the nucleus @ the center

13.2 Electron Pair RepulsionUnshared pr is pear-shaped w/ stem

end @ nucleus◦Acted upon by 1 nucleus

Shared pr is more slender bec it’s atracted by 2 nuclei◦Less repulsion bec it takes up less space◦lone prs – most repulsion – take up the

most space◦Repulsion betw unshared & shared pr is

intermediate

13.2 Electron Pair RepulsionCH4, NH3, H2O, & HF – all have 4

clouds around them◦\ expect all 4 clouds to pt to corners

of tetrahedronCH4 – all clouds are shared prs –

size & repulsions are =◦\ bond angle is 109.5o – perfect

tetrahedron◦Shape of molecule is tetrahedral

13.2 Electron Pair RepulsionNH3 – 1 lone pr & 3 shared prs –

since lone pr occupies more space, shared pairs are pushed together◦\ bond < is 107o

◦E- clouds form tetrahedron; but atoms of the molec for trigonal pyramid

◦Shape of molecule is trigonal pyramidal

13.2 Electron Pair RepulsionH2O – 2 unshared prs & 2 shared

prs◦Add’l cloud size of unshared prs

causes even greater reduction in bond < - 104.5o

◦E- clouds are tetrahedral, but molec is bent

HF – only 1 bond axis, \ no bond angle◦180o – molec is linear

The diff in molec shape results from unequal space occupied by unshared prs & bonds (shared prs)

13.3 Hybrid OrbitalsThe 2nd model of molec shape

considers the diff ways 2 & p orbitals ma overlap when e-’s are shared◦C has 4 outer e-’s Expect 2 half-filled p orbitals avail for

bonding

13.3 Hybrid OrbitalsHowever, C undergoes hybridization

during bondingThe 1s orbital & 3 p orbitals combine into 4

equivalent hybrid orbitals. Called sp3 hybrids or hybrid orbitals The 4 orbitals are degenerate – same energy

Ea contains 1 e- The sp3 hybrids are arranged in

tetrahedral shapeEa can bond to another atom

If ea bonds to an identical atom, the 4 bonds are equivalent

13.4Geometry of Carbon CompoundsMethane – CH4 – 1 C atom & 4 H

atoms◦Bonds involve s orbital of ea H atom

w/ 1 sp3 hybrid orbital of C 109.5o betw ea C – H bond axis

C exhibits catenation◦Occurs when 2 C atoms bond w/ ea

other by overlap of an sp3 orbital from ea C atom Other sp3 orbitals may bond w/ s orbital

of H

13.5 Sigma & Pi BondsA covalent bond is formed when

2 orbitals from diff atoms overlap & share an e- pair

Sigma Bond (s)– formed when the 2 orbitals that overlap lie directly on the bond axis ◦Overlap end-to-end or “head-on”

13.5 Sigma & Pi BondsDifferent ways to form a sigma

bond:1. 2 s orbitals2. An s & a p orbital3. 2 p orbitals (overlapping end-to-

end)4. 2 hybrid orbitals ex) sp3’s5. A hybrid orbital & an s orbital

13.5 Sigma & Pi BondsSince p orbitals are not spherical,

when 2 half-filled p orbitals overlap, they can form 1 of 2 types of bonds1. Overlap end-to-end & form a s

bond2. Overlap sideways (parallel) & form

a Pi Bond (p)

13.5 Sigma & Pi BondsEthylene (ethene, C2H4) shows

both types of bonding◦In both C atoms, 3 orbitals hybridize 1 s & 2 p form 3 sp2 orbitals

Lie in the same plane ~ 120o bond angle The 3rd p orbital does not hybridize

Perpendicular to plane of sp2 orbital◦An sp2 orbital from ea C atom

overlaps end-to-end - s bond

13.5 Sigma & Pi BondsThe 2 remaining sp2 orbitals

from ea C atom bond w/ 2 separate H atoms◦sp2 to s s bond

The unhybridized p orbitals overlap sadeways - p bond

C atoms have a s bond & a p bond betw them◦Double bond – 2 prs of e-’s are

shared

13.5 Sigma & Pi BondsAcetylene (ethyne)

◦1 s & 1 p orbital hybridize to form and sp hybrid orbital in ea C atom Leaves 2 p orbitals perpendicular to ea

other & perpendicular to the sp hybrids An sp from ea C overlap to for a s bond 2 p orbitals from ea C ovrlap to form 2 p

bonds◦\ acetylene has 1 s & 2 p bonds

betw C atoms Triple bond – 3 shared prs of e-’s

13.5 Sigma & Pi BondsDouble & triple bonds are less flexible,

shorter, & stronger than a single bondp bonds – easier to break bec e-’s

forming bond are farther from nuclei◦\ molecs containing multiple bonds are

usually more reactive than similar molecs w/ only a single bond

Unsaturated Comps – comps which contain double or triple bonds betw C atoms

13.6 Organic NamesNames for organic comps have a

suffix which describes how the atoms are bonded◦Comps ending in a n e have all single

bonds betw C atoms Saturated Comps - CnH2n+2

◦Comps ending in e n e have a double bond betw C atoms

◦Comps ending in y n e have a triple bond betw C atoms

13.6 Organic NamesPrefixes show # of C atoms in chain

or ring◦H2C CH2 - ethene

◦H C C H - ethyneMolecs whose C atoms form a ring

begin w/ cyclo◦Simplified diagrams can be used to

represent cyclic comps C atoms are @ the vertices

13.6 Organic NamesC forms 4 bonds

◦4 single bonds◦1 double & 2 single bonds◦1 triple & 1 single bond◦2 double bonds

\ assume a C atom has enough H atoms bonded to it to give it 4 bonds

13.7 Multiple Bond Molecular ShapesFormaldehyde contains a double

bond betw C & O

13.7 Multiple Bond Molecular ShapesN2 contains a triple bond

13.7 Multiple Bond Molecular ShapesUsing VESPR Theory we can still

predict the shapes of molecs containing multiple bonds◦A double bond occupies more space

than a single bond 4 e-’s betw bonded atoms instead of 2

◦Triple bond occupies even more space 6 e-’s

13.7 Multiple Bond Molecular ShapesIn formaldehyde –

◦3 clouds around C atom 2 single & 1 double bond No unshared prs; assume trigonal planar

shape; 120o

However, since double bond takes up more space than single bonds, H-C-H bond angle is less than 120o - 116o

The H – C – O bond angle is more than 120o

122o

13.7 Multiple Bond Molecular ShapesWhen C has 2 double bonds, the

molec will be linearCO2 -

13.7 Multiple Bond Molecular ShapesKetene:

◦ 2 dbl bonds on 1 C atom – that part is linear

◦ Other C atom has 2 single & 2 dbl bond like formaldehyde

13.7 Multiple Bond Molecular ShapesWhen C is triple bonded to

another atom, molec is linear

13.7 Multiple Bond Molecular ShapesIn most comps, outer level is

considered full w/ 8 e-s◦If outer level is 3rd or higher, atom

can contain > 8 e-’s Mostly nonmetals (usually halogens) form

comps w/ outer level containing 10, 12, or 14 e-’s This is how Noble Gases react

13.8 Benzene C6H6 One of the tip 20 industrial

chemicals in US◦Used in drugs, dyes, solvents◦Highly toxic & a carcinogen

Ea C atom in the benzene ring has 3 sp2 hybrids & 1 p orbital◦sp2 orbital from ea of the 6 C atoms

overlap & form a ring of 6 s bonds

13.8 Benzene C6H6 ◦p orbitals overlap sideways & form

ring of p bonds◦Left over sp2 orbital from ea C

overlaps w/ s orbital from H atom

13.8 Benzene C6H6 One main characteristic of

benzene is the p e-’s can be shared among all C atoms◦- delocalized◦Delocalization causes greater

stability in benzene

13.8 Benzene C6H6 Many ways to represent benzene:

Represent delocalized e-s from p bonds

13.8 Benzene C6H6 Conjugated system – group of

atoms which contain multiple p overlap◦Multiple p bonds◦Multiple double or triple bonds

◦ C C C C

◦Conjugated systs add special stability to the molecs

13.9 IsomersIsomerism – the existence of 2 or

more subst w/ the same molecular formula, but diff stuctures◦These structures are isomers Very common in organic chem

13.9 IsomersC4H10 – butane – 2 structures

can be drawn for this formulaButane methyl propane

(isobutane)

◦These are structural isomers or skeleton isomers – C chain is altered

13.9 IsomersGeometric isomers – coposed of

the same atoms bonded in the same order, but w/ diff arrangement of atoms around a double bond◦p bond prevents atoms from rotating

w/ respect to ea other◦A diff arrangement around a dbl

bond since rotation is not possible

13.9 Isomers◦Cis 2 butene

◦Trans 2 butene

13.9 IsomersCis – the CH3 group (or anything

other than H) are next to ea other (on same side)

Trans - the CH3 group (or anything other than H) are on opposite side (across)

13.9 IsomersPositional Isomers – occurs w/ a

3rd elem or mult bond where the 3rd elem or mult bond can occupy 2 or more diff positions

Functional Isomers – Formed when a 3rd elem can be bonded in 2 diff ways

A mass spectrometer can be used to distinguish betw isomers having similar props.◦Uses charge to mass ratios of ion

fragments

13.10 Inorganic CompoundsHybridize like organic comps

◦Be ends in 2s2 – hybridizes 2 orbitals 2 sp orbitals

Linear molec

13.10 Inorganic CompoundsB ends in 2s2 2p1 - 3 orbitals

hybridize◦3 sp2 orbitals

Trigonal planar

13.11 Bond summaryBCl3 – trigonal planar

◦Used to produce high-purity metalsHigher atomic mass elems tend

to hybridize their bonding orbitals much less than lighter elems do◦May be bec heavier atoms can have

more bonded atoms around them bec they are larger.