24 burdge organic lecture notes.pdf

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Chemistry: Atoms First

Julia Burdge & Jason Overby

Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Chapter 24

Organic Chemistry

Kent L. McCorkle

Cosumnes River College

Sacramento, CA

Organic Chemistry

24

24.1 Why Carbon is Different 24.2 Classes of Organic Compounds

Basic Nomenclature

Molecules with Multiple Substituents Molecules with Specific Functional Groups

24.3 Representing Organic Molecules

Condensed Structural Formulas Kekulé Structures Skeletal Structures Resonance

24.4 Isomerism

Constitutional Isomerism Stereoisomerism

24.5 Organic Reactions

Addition Reactions Substitution Reactions Other types of Organic Reactions

24.6 Organic Polymers

Addition Polymers Condensation Polymers Biological Polymers

Why Carbon is Different

Organic chemistry is the study of compounds that contain carbon

and hydrogen.

Examples of organic compounds include:

Methane, CH

4

Ethanol, C

2

H

5

OH

Ascorbic acid, C

5

H

7

O

4

COOH

Methyl amine, CH

3

NH

2

Carbon tetrachloride, CCl

4

24.1

Why Carbon is Different

Carbon tends to complete its octet by sharing electrons:

Electron configuration effectively prohibits ions formation.

[He]2s

2

2p

2

Intermediate electronegativity (2.5)

Carbon forms four covalent bonds

Carbon forms strong carbon-carbon bonds due to small size.

Hybridization and small size result in relatively strong

π

bonds.

Carbon has no

d

electrons or orbitals that are susceptible to attack.

Why Carbon is Different

Carbon forms chains containing single, double and triple

carbon-carbon bonds.

Each carbon atom in a compound is classified by the number of

other carbon atoms bonded to it.

Primary

(1

°

) – bonds to one other carbon

Secondary

(2°)

– bonds to two other carbons

Tertiary

(3°)

– bonds to three other carbons

Quaternary

(4

°

)

– bonds to four other carbons

1°carbon 2°carbon 3°carbon

4°carbon

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Why Carbon is Different

Organic compounds that are related to benzene or contain benzene

rings are called

aromatic

compounds.

Organic compounds that do not contain the benzene ring are called

aliphatic

compounds.

Classes of Organic Compounds

A variety of different types of organic compounds result from the

following:

1)

Carbon’s ability to form chains by bonding with itself

2)

The presence of elements other than carbon and hydrogen

3)

Functional groups

4)

Multiple bonds.

24.2

An

alkyl group

is a portion of a molecule that resembles an alkane.

An

alkyl group

is a portion of a molecule that resembles an alkane.

A functional group is a group of atoms that determines many of a

molecule’s properties.

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A functional group is a group of atoms that determines many of a

molecule’s properties.

Many compounds contain more than one functional group.

An

amino acid

contains both an amine group and the carboxyl

group.

carboxyl group amine group

alanine

Worked Example 24.1

Many familiar substances are organic compounds. Some examples include aspartame, the artificial sweetener in the sugar substitute Equal and in many diet sodas; salicylic acid, found in some acne medicines and wart-removal treatments; and amphetamine, a stimulant used to treat narcolepsy, attention-deficit hyperactivity disorder (ADHD), and obesity. Identify the functional group(s) in each molecule.

StrategyLook for and identify the combination of atoms show in Table 24.2.

Worked Example 24.1 (cont.)

Solution(a) From left to right, aspartame contains a –COOH (carboxy) group, an –NH2group (amine),a –CONHR group (amide), and a –COOR group (ester).

(b) Salicylic acid contains an –OH group (hydroxy) and a –COOH group (carboxy).

(c) Amphetamine contains an –NH2group (amine).

Worked Example 24.1 (cont.)

Think About ItIn part (b), the salicylic acid molecule contains a benzene ring and is therefore aromatic. When the hydroxy group is attached to a benzene ring, the resulting aromatic compound is a phenol, not an alcohol. Amphetamine has several legitimate medicinal uses, but it is also one of the most commonly misused drugs in the United States. Because it frequently is prescribed to adolescents for ADHD, much of it finds its way into high schools, where its misuse is a serious problem. A closely related compound that frequently makes headlines is methamphetamine.

In August 2005, an issue of Newsweekmagazine devoted a cover story to methamphetamine and its abuse.

Name the compound shown below:

Step 1:

The longest continuous carbon chain contains five C atoms.

The longest continuous carbon chain could be identified in another way:

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Step 2:

Number the carbon atoms beginning at the end nearest the

substituent (shaded in green):

Step 3:

The substituent is a methyl group, -CH

3

. It is attached to

carbon 2.

2-methylpentane

H

C

Cl

C

H

Solution:

Step 1:

Name the parent alkane.

The longest chain contains five carbons.

The parent alkane is pentane.

Solution:

Step 2:

Number of the carbons to give the lowest number for chlorine.

The compound is 3-chloropentane.

H

C

Cl

C

H

1 2 3 4 5

H

C

Cl

C

H

Worked Example 24.2

Give names for the following compounds:

(a) (b) (c)

StrategyUse the three-step procedure for naming substituted alkanes: (1) name the parent alkane, (2) number the carbons, and (3) name and number the substitutent. (Consult Table 5.8 for parent alkane names.)

Worked Example 24.2 (cont.)

Solution(a) This is a five-carbon chain. We can number the carbons starting at either end because the Cl substituent will be located on carbon 3 either way:

The name is 3-chloropentane.

(b) This may look like a substituted pentane, too, but the longest chain in this molecule is seven carbons long.

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Worked Example 24.2 (cont.)

Solution(b) Although Lewis structures appear to be flat and to contain 90 angles, the C atoms in this molecule are all sp3_{-hybridized (four electron domains} around each) and there is free rotation about the C−C bonds. Thus, the molecule can also be drawn as

The substituent is a methyl group on carbon 4. The name is 4-methylheptane.

(c) This is a substituted hexane.

The name is 2-methylhexane.

Think About ItA common error is to misidentify the parent alkane. Double-check to be sure you have identified the longest continuous chain in the molecule. Also be sure to number the carbon atoms so as to give the substituent the lowestpossible number.

Representing Organic Molecules

Representing organic molecules is important because the atoms

may be arranged in an enormous variety of ways.

A

condensed structural formula

or

condensed structure

shows the

same information as a structural formula, but in a condensed form.

24.3

C

8

H

18

CH

3

CH

2

CH

2

CH

2

CH

2

CH

2

CH

2

CH

3

CH

3

(CH

2

)

6

CH

3

Molecular

Formula

Structural

Formula

Condensed Structural

Formula

Representing Organic Molecules

A

condensed structural formula

or

condensed structure

shows the

same information as a structural formula, but in a condensed form.

C

8

H

18

CH

3

CH(CH

3

)(CH

2

)

4

CH

3

Molecular

Formula

Structural

Formula

Condensed Structural

Formula

CH

3

CHCH

2

CH

2

CH

2

CH

2

CH

3

CH

3

Kekulé structures

are similar to Lewis structures, except that they

do not show lone pairs.

Skeletal structures

consist of straight lines that represent

carbon-carbon bonds.

Carbon atoms are not shown.

Hydrogen atoms attached to carbon are not shown.

Heteroatoms

are atoms other than carbon or hydrogen.

If hydrogen is attached to a heteroatom, it must be included in the

skeletal structure.

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Worked Example 24.3

Write a molecular formula and a structural formula (or condensedstructural formula) for the following:

(a) (b)

StrategyCount the C atoms represented and the heteroatoms shown. Determine how many H atoms are present using the octet rule. Each line represents a bond. (Double lines represent double bonds.) Count one C atom at the end of each line unless another atom is shown there. Count the number of H atoms necessary to complete the octet of each C atom.

C atom + 1 H atom C atom + 3 H atoms

C atom + 3 H atoms C atom + 1 H atom

C atom + 3 H atoms

C atom (no H atoms)

Worked Example 24.3 (cont.)

Solution(a) Molecular formula: C4H8; structural formula: CH3(CH2)2CH3.

(b) Molecular formula: C2H5NO; structural formula: CH3CONH2.

Think About ItMake sure that each C atom is surrounded by four electron pairs: four single bonds, two single bonds and a double bond, or two double bonds. Remember that the single bonds to H typically are not shown in a skeletal structure–you have to remember that they are there and account for the H atoms when you deduce the formula.

Resonance

Chemists sometimes use curved arrow to specify the differences in

positions of electrons in resonance structures.

Worked Example 24.4

Following the curved arrows, draw a resonance structure for the HCONH-_ion.

StrategyMove electron pairs as indicated by the curved arrows, and calculate formal charges to reposition the negative charge.

SolutionTo determine the formal charge, we must first calculate the number of valence electrons on the atoms in question. The O and N atoms have sixand five valence electrons, respectively.

The formal charge on N, which had been −1, is now [5 – (3 + 2)] = 0. The formal charge on O, which had been 0, is now [6 – (1 + 6)] = −1. Thus, the negative charge now resides on the oxygen atom.

Think About ItCount the number of electrons pairs around each atom. Remember that the C and N atoms can have no more than four electron pairs. The H atom can have no more than one electron pair.

Worked Example 24.5

Adenosine triphosphate (ATP) is sometimes called the “universal energy carrier” or “molecular energy currency.” It contains two high-energy bonds (shown in red) that, when hydrolyzed(broken by the addition of water), release the energy necessary for cell function. Resonance stabilization of the hydrogen phosphate ion is one of the reasons the breakdown of ATP releases energy.

Draw all the possible resonance structures for the hydrogen phosphate ion (HPO42-). Use curved arrows to indicate how electrons are repositioned, and determine the position(s) of the negative charges.

StrategyDraw a valid Lewis structure for HPO42-, and determine whether and where electrons can be repositioned to produce one or more additional structures. Indicate the movement of electrons with curved arrows, and draw all possible resonance structures. Calculate the formal charge on each atom to determine the placement of charges.

A valid Lewis structure for the hydrogen phosphate ion is

For the purpose of determining formal charges, P and O have five and six valence electrons, respectively.

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Worked Example 24.5 (cont.)

SolutionA lone pair can be moved from one of the oxygen atoms to create a double bond to the phosphorus, and a pair of electrons from the originaldouble bond can be moved onto thatoxygen atom. The net result is simply a repositioning of the double bond by moving two electron pairs. This can be done once more, giving a total of three resonance structures for HPO42-. In each of the resonance structures, the formal charge on phosphorus is [5 – (5)] = 0. The formal charge on each singly bonded oxygen is [6 – (1 + 6)] = –1, and the formal charge on the doubly bonded oxygen is [6 – (2 + 4)] = 0.

Think About ItATP can also be hydrolyzed to give AMP (adenosine monophosphate) and pyrophosphate(P2O74-). Pyrophosphate hydrolyzes, in turn, to give two hydrogen phosphate ions. The oxygen atoms that can help delocalize the negative charges are highlighted.

These structures can also be drawn with one double bond to each phosphorus atom, to minimize formal charges.

Isomerism

Isomers are different compounds that have the same chemical

formula.

Constitutional isomerism

occurs when the same atoms can be

connected in two or more different ways.

24.4

Isomerism

Stereoisomers

are those that contain identical bonds but differ in

the orientation of those bonds in space.

There are two types of stereoisomers: geometrical and optical

isomers.

Geometrical isomers

occur in compounds that have restricted

rotation around a bond.

Compounds that form double bonds can form geometrical isomers.

Isomerism

Optical isomers

are stereoisomers that are mirror images of each

other, but are not superimposable.

Isomerism

Molecules with nonsuperimposable mirror images are

chiral

.

A pair of molecules that are mirror images of each other are called

enantiomers

.

Most chemical properties of enantiomers and all of their physical

properties are identical.

Isomerism

Three dimensional molecules are represented on paper using solid

lines, dashes and wedges.

Solid lines represent bonds that lie in the plane of the page.

Dashes represent bonds that point behind the page.

Wedges represent bonds that point in front of the page.

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Isomerism

Enantiomers are

optically active.

If the plane of polarization is rotated to the right, the isomer is

dextrorotary.

If the plane of polarization is rotated to the left, the isomer is

levorotary.

In an equimolar mixture of both enantiomers, called a

racemic

mixture

, the net rotation is zero.

Organic Reactions

An

electrophile

is a species with a positive or partial positive

charge.

Electrophiles are “electron loving.”

Electrophiles are attracted to a region of negative charge or a partial

negative charge.

Electrophiles are electron poor.

24.5

Organic Reactions

A

nucleophile

is a species with a negative or partial negative

charge.

Nucleophiles “love a nucleus.”

Nucleophiles are attracted to a region of positive charge or a partial

positive charge.

Nucleophiles are electron rich.

Organic Reactions

An

addition reaction

is one that forms a new bond to a carbon.

In an

electrophilic addition

, an electrophile attacks a region of

electron density:

Organic Reactions

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Organic Reactions

In a

nucleophilic addition

, a bond forms when a nucleophile

donates a pair of electrons to an electron-deficient atom.

Organic Reactions

A

substitution reaction

, occurs when one group is replaced by

another group.

Electrophilic substitution occurs when an electrophile attacks an

aromatic molecule and replaces a hydrogen atom:

Organic Reactions

Nucleophilic substitution occurs when a nucleophile replaces

another group on a carbon atom:

Worked Example 24.6

The electrophilic addition of water to a molecule containing a double bond, known as hydration, is a reaction that is important biologically. One example is the enzyme-catalyzed hydration of fumarate to form malate, one of the steps in the citric acid cycle (also known as the Krebs cycle).

Using curved arrows to indicate the movement of electrons, draw the mechanism for the electrophilic addition of water to fumarate to yield malate.

Worked Example 24.6 (cont.)

StrategyFor electrophilic addition, draw Lewis structures with the electrophile close to the region of electron density where attack will occur.

SolutionThe electrophile is one of the H atoms in H2O. The site of attack is the pi bond between the C atom in fumarate:

Think About ItThis time the product is the same regardless of which C atom forms the C−O bond and which forms the C−H bond, but this is not always the case. If CH3CHCH3undergoes hydration, for instance, there are two possible products:

In this case, for reasons beyond the scope of this book, the product that forms is 2-propanol. If you take organic chemistry, you will learn what factors determine which C atom gets the −OH group and which gets the −H.

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Worked Example 24.7

Using curved arrows to indicate the movement of electrons, draw the mechanism for each of the following reactions: (a) nucleophilic addition of CN-_{to CH}

3CHO and (b) electrophilic substitution of benzene with +_SO

3H. (Draw all resonance structures for the carbocation intermediate.)

StrategyFor nucleophilic addition, draw Lewis structures with the nucleophile close to the electron-poor atom where attack will occur (in this case, the carbonyl carbon). For electrophilic substitution, draw Lewis structures with the electrophile close to the site of attack on the benzene ring. Remember that nucleophiles are attracted to and react with electron-poor atoms whereas electrophiles are attracted to and react with the electron-rich areas of the molecule. Using this information, and the octet rule, determine which electrons are likely to be involved in the reaction and indicate their repositioning with curved arrows.

Worked Example 24.7 (cont.)

Solution(a) The nucleophile is CN-_{. The site of attack is the carbonyl C in} CH3CHO, which is electron-poor because it is bonded to the more electronegative atom.

(b) The electrophile is +_SO

3H. The site of attack is the electron-rich, delocalized pi bonds of the benzene ring.

Think About ItAdditions such as the one in part (a) are carried out in acidic solution. The availability of protons in solution makes the final step the protonation of the negatively charged O atom:

Also, the C atoms in benzene are all equivalent, so the choice of which C bear the substituent in part (b) is arbitrary. All the following represent the same product:

Organic Reactions

An

elimination reaction

is one in which a double bond forms and a

molecule such as water is removed.

Oxidation-reduction reactions

involve the

loss

and

gain

of

electrons.

When a molecule gains O or loses H, it is

oxidized.

Organic Reactions

An

isomerization reaction

is one in which one isomer is converted

to another.

Organic Polymers

Polymers

are molecular compounds that are made up of many

repeating units.

The repeating units in a polymer are called

monomers

.

Polymers typically have very high molar masses.

24.6

monomer

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Organic Polymers

Addition polymers

form when monomers such as ethylene join end

to end to make polyethylene.

Addition polymers form according to the following mechanism:

Step 1

A

radical

—a species that contains an unpaired electron—

attacks a carbon atom on an ethylene molecule.

Organic Polymers

Addition polymers

to end to make polyethylene.

Addition polymers form according to the following mechanism:

Steps 2 - 4

The double bond breaks, and a bond forms between the

original radical and the ethylene molecule. A new radical forms.

Organic Polymers

Addition polymers

Step 5

The new radical attacks another ethylene molecule forming

an another radical in a step called

propagation.

Organic Polymers

Addition polymers

Step 6

Termination

occurs until the radicals encounter another

radical.

Organic Polymers

Condensation polymers

form when molecules with two different

functional groups combine, with the elimination of a small

molecule, often water.

Organic Polymers

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Organic Polymers

A

copolymer

is made up of two or more

different

molecules.

Organic Polymers

Naturally occurring polymers include

proteins, polysaccharides,

and

nucleic acids.

Proteins

are polymers of amino acids.

The bonds that form between amino acids are called

peptide bonds

.

Very long chains of condensed amino acids are called

proteins

.

Shorter chains of condensed amino acids are called

polypeptides

.

Organic Polymers

Polysaccharides

are polymers of sugars such as glucose and

fructose.

Organic Polymers

Nucleic acids

are polymers of

nucleotides.

Two important nucleic acids are

deoxyribonucleic acid (DNA)

and

ribonucleic acid (RNA)

.

Each

nucleotide

consists of a

base,

a furanose sugar, and a

phosphate group.

Chapter Summary: Key Points

24

Unique features of carbon Classes of organic compounds Naming organic compounds Isomerism

Constitutional isomerism Stereoisomerism

Geometrical isomers Optical isomers Organic reactions

Addition reactions Electrophilic addition Nucleophilic addition Substitution reactions

Electrophilic substitution Nucleophilic substitution Elimination reactions Oxidation-reduction reactions Isomerization reactions

Polymers Addition Condensation Biological

Proteins Carbohydrates Nucleic Acids

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