Chemical StructureA Lewis structure shows the symbol of the element surrounded by a number of dots equal to the number of electrons in the outer shell of an atom of that element. In

SCH 102

Dr. Solomon Derese 77

Chemical Structure

… structure determines properties …

SCH 102

Dr. Solomon Derese

Chemical structure is the key to everything inchemistry. The properties of a substance depend onthe atoms it contains and the way these atoms areconnected.

78

In this unit we will study the three important featuresof the structures of organic compounds:

I. Lewis structure,II. Shape (Hybridization) andIII. Spatial arrangement of atoms in a molecule

(Stereochemistry)

SCH 102


Lewis StructuresDefinitionChemical structures in which all the electrons in thevalence shells are shown as dots (ꓺ) or solid lines (▬)are called Lewis structures. It shows how valenceelectrons are arranged among atoms in a molecule.

A Lewis structure shows the symbol of the elementsurrounded by a number of dots equal to the numberof electrons in the outer shell of an atom of thatelement. In Lewis structures, the atomic symbolrepresents the core; that is, the nucleus and all innershell electrons.

SCH 102


There are three general rules for drawing Lewisstructures.

1. Draw only the valence electrons.2. Give every second-row element no more than

eight electrons (octet rule).3. Give each hydrogen two electrons (duet rule).

When drawing a Lewis structure, make sure hydrogenatoms are surrounded by two electrons and C, O, N,and halogen (F, Cl, Br, I) atoms are surrounded byeight electrons, obey octet rule.

SCH 102


Skills in writing Lewis structures formolecules is fundamental to theunderstanding of organic chemistry.

Valence electrons not used in bondingare called nonbonding electrons, lone-pair electrons, or simply, lone pairs.

SCH 102


H has one valence electron and F has seven.

or2 e’s around H 8 e’s around F

3 lone pairs of e’s on F

Let us draw the Lewis structure for HF

The resulting molecule gives both H and F a filledvalence shell.

Both H and F donate one electron to form a two-electron bond

a two-electron bond

SCH 102


In a Lewis structure, a solid line indicates a two-electron covalent bond.The usual number of bonds and nonbondedelectron pairs of common neutral atoms aresummarized below.

# of bonds

# of nonbondedelectron pairs

1 4 3 2 1

0 0 2 2 3

nonbonded electron pairs

SCH 102


Steps of Drawing a Lewis StructureStep 1: Arrange atoms next to each other that you

think are bonded together. Put the leastelectronegative atom in the centre

Always place hydrogen atoms and halogen atoms onthe outside because H and X (X = F, Cl, Br, and I) formonly one bond.

C HH

HH

C HH

H Hnot

This H cannot form two bonds

For CH4

SCH 102


As a first approximation, place no more atoms aroundan atom than the number of bonds it usually forms.

For CH5N:

C NH

HH

notH

H C NH

HH H

H

4 atoms around C 3 atoms around N

SCH 102


Step 2: Count the number of valence electrons for eachatom in the molecule.

• Count the number of valence electrons from allatoms.

• Use the periodic table to figure out how manyelectrons are available in the valence electrons.

• Add one electron for each negative charge.• Subtract one electron for each positive charge.• This sum gives the total number of electrons that

must be used in drawing the Lewis structure.

SCH 102


Step 3: Arrange the electrons around the atoms.

• Place a bond between every two atoms, giving twoelectrons to each H and no more than eight to anysecond-row atom.

• Use all remaining electrons to fill octets with lonepairs.

• If all valence electrons are used and an atom doesnot have an octet, form multiple bonds.

Step 4: Assign formal charges to all atoms.

SCH 102


Example 1: Draw a Lewis structure for methane, CH4.

Place C in the center and 4 H’s on the outside.

Step 2: Count the total number of valence electrons

C HH

HH

Step 1: Arrange the atoms

1C x 4e- = 4e-

4H x 1e- = 4e-

Total = 8e-

SCH 102


Step 3: Add the bonds and lone pairs electrons.

Adding four two-electron bonds around carbon usesall eight valence electrons, and so there are no lonepairs.

C HH

H

H

C HH

H

H

Add a bond betweeneach C and H

2e- s around H

8e- s around C

SCH 102


To check whether a Lewis structure is valid, we must answer YES to three questions:

• Have all the electrons been used?• Is each H surrounded by two electrons?• Is each second-row element surrounded by no

more than eight electrons?

The answer to all three questions is YES for methane,so the Lewis structure drawn for CH4 is valid.

SCH 102


Example 2: Draw a Lewis structure for methanol, acompound with molecular formulaCH4O.

C OH

H

H


H• H on the outside• Four atoms around C• Two atoms around O

Step 2: Count the total number of valence electrons1C x 4e- = 4e-

1O x 6e- = 6e-

4H x 1e- = 4e-

Total = 14e-

SCH 102



C OH

H

H

H

Add a bond betweeneach atoms

C O

H

H

H

HAdd lone

pairs on OC O

H

H

H

H

Only 10 e- s used

No octet

Valid structure

..

..

This uses all 14 electrons,giving every H two electronsand every second-row elementeight, C and O in this case.

SCH 102


Example 3: Draw a Lewis structure for eachcompound C2H6.

C CH


H

Step 2: Count the total number of valenceelectrons

2C x 4e- = 8e-

4H x 1e- = 4e-

Total = 12e-

HH

SCH 102



C CH H

HHOnly 10 e- s used

Add lone pairs of e-s

C CH H

HH

..

To give both C's an octet, change one lone pair intoone bonding pair of electrons between the two C's,forming a double bond.

C CH H

HH

..C CH H

HH

Move a lone pair

All 12 e- s used

No octet

Each C has 4 bonds

SCH 102


Example 3: Draw a Lewis structure for eachcompound C2H2.

C CH


H

Step 2: Count the total number of valence electrons

2C x 4e- = 8e-

2H x 1e- = 2e-

Total = 10e-

SCH 102



C CH H

Only 6 e- s used

Add lone pairs of e-s

C CH H..

To give both C's an octet, changetwo lone pairs into two bondingpairs of electrons, forming atriple bond.

All 10 e- s used

No octet

..

Move a lone pair

C CH H..

No octetC CH H

Each C has 4 bonds

SCH 102


After placing all electrons in bonds and lone pairs, usea lone pair to form a multiple bond if an atom doesnot have an octet.

Change one lone pair into one new bond for each twoelectrons needed to complete an octet.

The examples discussed above are for neutralcompounds. However, compounds can also be eitherpositively or negatively charged and we should beable to draw the structures of such compounds andassign charges to each individual atom in theparticular compound.

SCH 102


The decision as to where to put the charge is made bycalculating the formal charge for each atom in an ionor a molecule.Formal charge is the charge assigned to individualatoms in a Lewis structure.

Formal charge for an atom is calculated using theformula:

Formal charge = # of valence e− s − (# of nonbonding e− s + of bonding e- s)

SCH 102


A species containing a positively charged carbon iscalled a carbocation, and a species containing anegatively charged carbon is called a carbanion. Aspecies containing an atom with a single unpairedelectron is called a radical (often called a free radical).

Carbocation Carbanion Free Radical

SCH 102


Example 4: Determine the formal charge on eachatom in the hydronium ion H3O+.

Formal charge = # of valence e− s − (# of nonbonding e− s + # of bonding e- s)

For H, # of valence e− s = 1, # of nonbonding e− s = 0 andnumber of bonding e− s = 2

Formal charge for H = 1 − (0 + x2) = 0

For O, # of valence e− s = 6, # of nonbonding e− s = 2 andnumber of bonding e− s = 6

Formal charge for O = 6 − (2 + x6) = +1

SCH 102


The overall charge on the ion is the sum of all theformal charges.In this case the over all charge on the hydroniumion is 0 + 0 + 0 + 1 = +1.The table lists the bonding patterns and resultingformal charges for carbon, nitrogen, and oxygen.

Atom # of valence 4e-sFormal charge

+1 0 -1

C

N

O

4

5

6

SCH 102


In drawing a Lewis structure for a molecule withseveral atoms, sometimes more than onearrangement of atoms is possible for a givenmolecular formula.For example, there are two acceptablearrangements of atoms for the molecular formulaC2H6O.

Isomers

Ethanol Dimethyl etherIsomers are different molecules having the samemolecular formula.

SCH 102


Ethanol and dimethyl ether are constitutionalisomers because they have the same molecularformula, but the connectivity of their atoms isdifferent. For example, ethanol has one C – C bondand one O – H bond, whereas dimethyl ether hastwo C – O bonds.

SCH 102


Exceptions to the Octet RuleMost of the common elements in organiccompounds—C, N, O, and the halogens—follow theoctet rule.Hydrogen is a notable exception, because itaccommodates only two electrons in bonding.

This is the case for beryllium (Group 2A) and boron(Group 3A). These elements do not have enoughvalence electrons to form an octet in a neutralmolecule.

Sometimes there are not enough electrons in anatom to provide octet of electrons around the centralatom

SCH 102


Lewis structures for BeH2 , BF3 and AlCl3 show thatthese atoms have only four and six electrons,respectively, around the central atom. There isnothing we can do about this! There simply aren’tenough electrons to form an octet.

There is nothing we can do about this! There simplyaren’t enough electrons to form an octet.

4e-s around Be 6e-s around B

Because the Be and B atoms have less than an octetof electrons, these molecules are highly reactive.

6e-s around Al

SCH 102


Aluminum trichloride is an example of a compound inwhich aluminum, the element immediately belowboron in Group 3A, has an incomplete valence shell.Because their valence shells are only partially filled,trivalent compounds of boron and aluminum exhibit ahigh reactivity with compounds that have extraelectrons, enabling them to fill their octets

SCH 102


These elements have empty d orbitals available toaccept electrons, and thus they may have morethan eight electrons around them.

Another exception to the octet rule occurs with someelements located in the third row and later in theperiodic table.

For organic chemists, the two most commonelements in this category are phosphorus and sulfur,which can have 10 or even 12 electrons aroundthem.

SCH 102


10 e-s around S 12 e-s around S 10 e-s around P

8 e-s around S

SCH 102

Dr. Solomon Derese

Resonance

109

Some compounds cannot be adequatelyrepresented by a single Lewis structure.Look at the acetate ion, CH3CO2

- for instance.

Acetate anion

The Lewis structure for the acetateion suggests that, the two C-Obonds are different, one of themsingle bond and negatively chargedand the other one double bond andneutral.

Experimental evidence however indicate the two C-O bonds are equivalent.

SCH 102


Both carbon–oxygen bonds in the acetate anionare 127 pm in length, midway between the lengthof a typical C-O single (135 pm) and a typicalC=O double bond (120 pm). Each oxygen atomhas some negative charge.

The Lewis structure does not depict this reality.

The experimental observations for the acetateanion are better represented by a picture in whichthe electrons are equally distributed between thetwo oxygens.

SCH 102


Acetate anion

Two structures may be drawn for the acetateanion, each having the same connectivitydiffering only in the location of the pair ofelectrons.

Resonance is theory developed to reconcileexperimental data with the implications of Lewisstructures for compounds with two or morestructures.

SCH 102


The individual structures are resonance contributorsto the structure of the acetate anion and it is picturedas a resonance hybrid of these two structures.These structures differ in the arrangement ofelectrons, not in the position of the atoms (which isalready determined following the rules of drawingLewis structure).The actual properties of the acetate anion cannot berepresented by any one of the Lewis structures takenalone, the experimental facts are represented bydrawing the two resonance contributors.

SCH 102


The two resonance contributors taken togetherindicate that each oxygen atom bears half of thecharge on an electron and that the C-O bonds are thesame length.

The resonance relationship among resonancestructures is indicated by a double-headed arrow (↔)between them.

12

12

SCH 102


The double-headed arrow means that molecule is an“average” of all contributing structures; the moleculeis said to be a resonance hybrid of these structures.

The symbol ↔ does not mean that the two forms arein equilibrium with each other. No reaction is impliedby the double-headed arrow. There is only onestructure for the acetate anion, which is a hybrid ofthe tow structures drawn.The only difference between resonance forms is theplacement of the p and nonbonding valenceelectrons.

SCH 102


Resonance structures are two or more forms of amolecule where the chemical connectivity is the samebut the electrons are distributed differently aroundthe structure.Resonance occurs when electrons can flow throughneighboring p systems (double or triple bonds).

Resonance theory was developed as an attempt tocorrect a fundamental defect in Lewis structures.Lewis structures show electrons as being localized;they either are shared between two atoms in acovalent bond or are unshared electrons belonging toa single atom.

SCH 102


In reality, electrons distribute themselves in the waythat leads to their most stable arrangement. Thissometimes means that a pair of electrons isdelocalized, or shared by several nuclei.Writing the various Lewis formulas that contribute toa resonance hybrid can be made easier by usingcurved/curly/pushing arrows to keep track ofdelocalized electrons.

Curved/Curly/Pushing arrow Tail Head

SCH 102


Curved arrows show the origin (tail) and destination(head) of a pair of electrons.

Use this electron pair to form a double bond

Move an electron pair to O

All resonance structures are enclosed in a squarebracket, to indicate they picture one singlemolecule or ion, not different species.

SCH 102


Resonance contributors are significant for manyother ions. The nitrate and carbonate areexample of species for which a single resonancestructure are not satisfactory.

NO3-

CO3-2

SCH 102


A very important aspect of resonance structures isthat they have implications for the stability of themolecule they represent. A molecule represented byresonance structures is more stable than the structurerepresented by any of the resonance contributors.

Benzene

Structures for uncharged molecules may also haveresonance contributors. Benzene is a good example.

All the C-C bonds are equivalent and due to resonancebenzene is a very stable molecule, more in SCH 202.

SCH 102


The resonance contributors shown for the fourexamples are equivalent to each other. This is notalways the case. For example two resonance

This resonance form has theNegative charge on C

This resonance form has theNegative charge on O

For example the two resonance structures drawn foracetone anion are not equivalent.

minor contributor major contributor

SCH 102


The true structure of the acetone anion is more like thatof the form that places the negative charge on theelectronegative O atom rather than on C.Resonance contributors with no separation of charge,with maximum number of covalent bonds and with octetsof electrons around each atom (except hydrogen)contribute the most to experimentally observedproperties.These resonance contributors are thus more importantthan and are known as major contributors.Those that have fewer covalent bonds and a separation ofcharge have less effect on the properties of the speciesand are called minor contributors.

SCH 102


These are the rules for writing and interpretations ofresonance contributors:

1.Resonance structures are written for compoundsthat are not adequately described by a single Lewisstructure.

2.Resonance contributors have the same connectivity.Only nonbonding electrons and electrons inmultiple bonds change locations from oneresonance contributor to another. The electrons insingle bond are not involved.

3.The nuclei of atoms in different resonancecontributors remain the same.

SCH 102


4. The structure of a molecule is the weightedaverage of its resonance structures. Whenresonance structures are identical, they are equallyimportant descriptions of the molecule. When tworesonance forms are nonequivalent, the actualstructure of the resonance hybrid resembles themore stable form more than it resembles the lessstable form.

SCH 102


5. Resonance hybrids are more stable than any of theLewis structures used to describe them. Moleculesdescribed by resonance structures are said to beresonance-stabilized. In other words, resonanceleads to stability. Generally speaking, the larger thenumber of resonance forms, the more stable asubstance is because its electrons are spread outover a larger part of the molecule and are closer tomore nuclei.

SCH 102


6. Resonance forms obey normal rules of valency. Aresonance form is like any other structure: theoctet rule still applies.

7. Individual resonance forms are imaginary, not real.The real structure is a composite, or resonancehybrid, of the different forms.

Not a valid resonance form

10 e- s on this C

SCH 102


Not all resonance structures contribute equally to aresonance hybrid. We describe four ways to predictwhich structure contributes more to the hybrid.

The following preferences will help you to estimatethe relative importance of the various contributingstructures. In fact, we can rank structures by thenumber of these preferences they follow. Those thatfollow the most preferences contribute the most tothe hybrid, and any structure that violates all four ofthese preferences can be ignored.

SCH 102


Preference I: Filled Valence ShellsStructures in which all atoms have filled valence shells(completed octets) contribute more than those inwhich one or more valence shells are unfilled.

Greater contribution:both C and O have

complete valence shells

Lesser contribution:C has only six electrons

in its valence shell

SCH 102


Preference 2: Maximum Number of Covalent BondsStructures with a greater number of covalent bondscontribute more than those with fewer covalentbonds.

Greater contribution:eight covalent bonds

Lesser contribution:seven covalent bonds

SCH 102


Preference 3: Least Separation of Unlike ChargesStructures that involve separation of unlike chargescontribute less than those that do not involve chargeseparation because separation of charges costsenergy.

Greater contribution:no separation ofunlike charges

Lesser Contribution:separation of unlike

charges

SCH 102


Preference 4: Negative Charge on a MoreElectronegative Atom

Structures that carry a negative charge on a moreelectronegative atom contribute more than thosewith the negative charge on a less electronegativeatom. Conversely, structures that carry a positivecharge on a less electronegative atom contributemore than those that carry the positive charge on amore electronegative atom. Following are threecontributing structures for acetone:

SCH 102


Lesser contribution Greater contribution Should not be drawn

Structure II makes the largest contribution to the hybrid.while I contributes less because it involves separation ofcharges and because carbon has an in complete octet.Nevertheless, on structure I, the more electronegative Oatom has the negative charge and the less electronegativeC atom has the positive charge. Structure III violates allfour preference rules and should not be drawn.

SCH 102


In the structures we’ve been drawing until now, aline between atoms has represented the twoelectrons in a covalent bond.

Writing Organic Structures

The most commonly used shorthand for drawingchemical structures is skeletal structure.

Drawing every bond and every atom is tedious,however, so chemists have devised severalshorthand ways for writing structures.

SCH 102


The rules for drawing skeletal structuresRule 1Carbon atoms aren’t shown. Instead, a carbon atomis assumed to be at each intersection of two lines(bonds) and at the end of each line.

Rule 2Hydrogen atoms bonded to carbon aren’t shown.Because carbon always has a valence of 4, wementally supply the correct number of hydrogenatoms for each carbon. Hydrogen atoms bonded toatoms other than carbon are shown.

Rule 3Atoms other than carbon and hydrogen are shown.

SCH 102


C C CC

CH

H H H

H

H

H

H

CC C C

CCH H H

HH

HHHHHH

H

becomes

becomes

becomes

becomes

Skeletal structure

SCH 102


HC

H

CCCC

H H

H

HHH

Cl

CBrH

C C H Cl

Br

becomes

becomes

Skeletal structure

Documents

Chemical StructureA Lewis structure shows the symbol of the element surrounded by a number of dots equal to the number of electrons in the outer shell of an atom of that element. In