Honors Chemistry Chap 13

13.1 Electron Distribution
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Consider 2 models of molec structure which account for their shape
1 st model takes into account the repulsive forces of e- pairs
2 nd 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 Distribution



It’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 Repulsion

Also called VESPR Theory
◦ Valence Electron Shared Pair Repulsion
Theory

13.2 Electron Pair Repulsion

One 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 Repulsion


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Repulsion 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 (180 o apart)

13.2 Electron Pair Repulsion
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
3 e- prs – axes of chg clouds will be 120 o apart
◦ Trigonal Planar
 E- prs lie in the same plane as the nucleus
4 prs – axes of chg clouds will be as far apart as possible – 109.5
o
◦ Tetrahedral
 Will not lie in the same plane
 4 faces; ea is an equilateral triangle w/ the nucleus
@ the center

13.2 Electron Pair Repulsion


Unshared 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 Repulsion
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
CH
4
, NH
3
, H
2
O, & HF – all have 4 clouds around them
◦ \ expect all 4 clouds to pt to corners of tetrahedron
CH
4
– all clouds are shared prs – size & repulsions are =
◦ \ bond angle is 109.5
o – perfect tetrahedron
◦ Shape of molecule is tetrahedral

13.2 Electron Pair Repulsion

NH
3
– 1 lone pr & 3 shared prs – since lone pr occupies more space, shared pairs are pushed together
◦ \ bond < is 107 o
◦ E- clouds form tetrahedron; but atoms of the molec for trigonal pyramid
◦ Shape of molecule is trigonal pyramidal

13.2 Electron Pair Repulsion



H
2
O – 2 unshared prs & 2 shared prs
◦ Add’l cloud size of unshared prs causes even greater reduction in bond < - 104.5
o
◦ E- clouds are tetrahedral, but molec is bent
HF – only 1 bond axis, \ no bond angle
◦ 180 o – molec is linear
The diff in molec shape results from unequal space occupied by unshared prs
& bonds (shared prs)

13.3 Hybrid Orbitals

The 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 Orbitals
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
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However, C undergoes hybridization during bonding

The 1s orbital & 3 p orbitals combine into 4 equivalent hybrid orbitals .
Called sp 3 hybrids or hybrid orbitals

The 4 orbitals are degenerate – same energy

Ea contains 1 e-
The sp 3 hybrids are arranged in tetrahedral shape

Ea can bond to another atom
If ea bonds to an identical atom, the 4 bonds are equivalent

13.4Geometry of Carbon
Compounds

Methane – CH
4
– 1 C atom & 4 H atoms
◦ Bonds involve s orbital of ea H atom w/ 1 sp 3 hybrid orbital of C

 109.5
o betw ea C – H bond axis
C exhibits catenation
◦ Occurs when 2 C atoms bond w/ ea other by overlap of an sp 3 orbital from ea C atom
 Other sp 3 orbitals may bond w/ s orbital of H

13.5 Sigma & Pi Bonds
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
A 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 Bonds

Different ways to form a sigma bond:
1.
2 s orbitals
2.
An s & a p orbital
3.
2 p orbitals (overlapping end-to-end)
4.
2 hybrid orbitals ex) sp 3 ’s
5.
A hybrid orbital & an s orbital

13.5 Sigma & Pi Bonds

Since p orbitals are not spherical, when 2 half-filled p orbitals overlap, they can form
1 of 2 types of bonds
1.
Overlap end-to-end & form a s bond
2.
Overlap sideways (parallel) & form a Pi
Bond ( p )

13.5 Sigma & Pi Bonds

Ethylene (ethene, C
2 of bonding
H
4
) shows both types
◦ In both C atoms, 3 orbitals hybridize
 1 s & 2 p form 3 sp 2 orbitals
 Lie in the same plane ~ 120o bond angle
 The 3 rd p orbital does not hybridize
 Perpendicular to plane of sp 2 orbital
◦ An sp 2 orbital from ea C atom overlaps endto-end s bond

13.5 Sigma & Pi Bonds
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
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The 2 remaining sp2 orbitals from ea C atom bond w/ 2 separate H atoms
◦ sp 2 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 Bonds

Acetylene (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 Bonds
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Double & triple bonds are less flexible, shorter, & stronger than a single bond p 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 Names

Names 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 - C n
H
2n+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 Names

Prefixes show # of C atoms in chain or ring
◦ H
2
C CH
2
- ethene

◦ H C C H - ethyne
Molecs 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 Names
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C 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
Shapes
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Formaldehyde contains a double bond betw C & O

13.7 Multiple Bond Molecular
Shapes
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N
2 contains a triple bond

13.7 Multiple Bond Molecular
Shapes
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Using 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
Shapes
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In formaldehyde –
◦ 3 clouds around C atom
 2 single & 1 double bond
 No unshared prs; assume trigonal planar shape;
120 o
 However, since double bond takes up more space than single bonds, H-C-H bond angle is less than
120 o - 116 o
 The H – C – O bond angle is more than 120 o
 122 o

13.7 Multiple Bond Molecular
Shapes
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When C has 2 double bonds, the molec will be linear
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CO
2
-

13.7 Multiple Bond Molecular
Shapes
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Ketene:
◦ 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
Shapes
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When C is triple bonded to another atom, molec is linear

13.7 Multiple Bond Molecular
Shapes
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In most comps, outer level is considered full w/ 8 e-s
◦ If outer level is 3 rd 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 C
6
H
6
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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 sp 2 hybrids & 1 p orbital
◦ sp 2 orbital from ea of the 6 C atoms overlap
& form a ring of 6 s bonds

13.8 Benzene C
6
H
6
◦ 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 C
6
H
6
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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 C
6
H
6
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Many ways to represent benzene:

Represent delocalized e-s from p bonds

13.8 Benzene C
6
H
6

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 Isomers
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Isomerism – 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 Isomers
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
C4H10 – butane – 2 structures can be drawn for this formula
Butane methyl propane
(isobutane)
◦ These are structural isomers or skeleton isomers – C chain is altered

13.9 Isomers
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Geometric 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 Isomers
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Cis – the CH
3 group (or anything other than H) are next to ea other (on same side)
Trans - the CH
3 group (or anything other than H) are on opposite side (across)

13.9 Isomers
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Positional Isomers – occurs w/ a 3 rd elem or mult bond where the 3 rd elem or mult bond can occupy 2 or more diff positions
Functional Isomers – Formed when a 3 rd 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 Compounds
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Hybridize like organic comps
◦ Be ends in 2s 2 – hybridizes 2 orbitals
 2 sp orbitals
 Linear molec

13.10 Inorganic Compounds
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B ends in 2s 2 2p 1 - 3 orbitals hybridize
◦ 3 sp 2 orbitals
 Trigonal planar

13.11 Bond summary
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BCl
3
– trigonal planar
◦ Used to produce high-purity metals
Higher 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.