mcmurry organic chemistry 6th edition chapter 3 (c) 2003 2 representations of structural formulas...

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

2

Representations of Structural Formulas

Dot formulas are more cumbersome to draw than dash formulas and condensed formulas

Lone-pair electrons are often (but not always) drawn in, especially when they are crucial to the chemistry being discussed

2.12 Drawing Chemical Structures


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Dash formulas Each dash represents a pair of electrons This type of representation is meant to

emphasize connectivity and does not represent the 3-dimensional nature of the molecule


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Condensed structures: C-H and C-C and single bonds aren't shown but understood If C has 3 H’s bonded to it, write CH3

If C has 2 H’s bonded to it, write CH2; and so on. The compound called 2-methylbutane, for example, is written as follows:

Horizontal bonds between carbons aren't shown in condensed structures—the CH3, CH2, and CH units are simply but vertical bonds are added for clarity


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Condensed Structural Formulas In these representations, some or all of the dash lines are omitted

In partially condensed structures all hydrogens attached to an atom are simply written after it but some or all of the other bonds are explicitly shown

In fully condensed structure all bonds are omitted and atoms attached to carbon are written immediately after it

For emphasis, branching groups are often written using vertical lines to connect them to the main chain


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Three-Dimensional Formulas

The conventions for this are: Bonds that lie in the plane of the paper are indicated

by a simple line Bonds that come forward out of the plane of the

paper are indicated by a solid wedge Bonds that go back out of the plane of the paper are

indicated by a dashed wedge


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Classes of Compounds

Classification based on functional group Three broad classes

Hydrocarbons Compounds containing oxygen Compounds containing nitrogen

=>


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Hydrocarbons

Alkane: single bonds, sp3 carbons Cycloalkane: carbons form a ring Alkene: double bond, sp2 carbons Cycloalkene: double bond in ring Alkyne: triple bond, sp carbons Aromatic: contains a benzene ring

=>


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Compounds Containing Oxygen

Alcohol: R-OH Ether: R-O-R' Aldehyde: RCHO

Ketone: RCOR'

CH3CH2 C

O

H

CH3 C

O

CH3

=>


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Cyclic ethers and ketones

OO

THF

(tetrahydrofuran)cyclopentanone


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Carboxylic Acids and Their Derivatives

Carboxylic Acid: RCOOH Acid Chloride: RCOCl Ester: RCOOR' Amide: RCONH2

C

O

OH

C

O

Cl

C

O

OCH3C

O

NH2

=>


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Cyclic esters

O

O

O

O

Cyclic ester (cyclic ether and cyclic ketone)


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Compounds Containing Nitrogen

Amines: RNH2, RNHR', or R3N

Amides: RCONH2, RCONHR, RCONR2

Nitrile: RCN

N

O

CH3

CH3 C N

=>


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Summary of Important Families of Organic Compounds


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Summary (cont.)


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3.2 Alkanes and Alkane Isomers

Alkanes: Compounds with C-C single bonds and C-H bonds only (no functional groups)

The formula for an alkane with no rings in it must be CnH2n+2 where the number of C’s is n

Alkanes are saturated with hydrogen (no more can be added

They are also called aliphatic compounds


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Alkane Isomers

CH4 = methane, C2H6 = ethane, C3H8= propane

The molecular formula of an alkane with more than three carbons can give more than one structure C4 (butane) = butane and isobutane

C5 (pentane) = pentane, 2-methylbutane, and 2,2-dimethylpropane

Alkanes with C’s connected to no more than 2 other C’s are straight-chain or normal alkanes

Alkanes with one or more C’s connected to 3 or 4 C’s are branched-chain alkanes


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Constitutional Isomers

Isomers that differ in how their atoms are arranged in chains are called constitutional isomers

They must have the same molecular formula to be isomers


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3.4 Naming Alkanes

Compounds are given systematic names by a process that uses Prefix-Parent-Suffix

Follows specific rules Named as longest possible chain Carbons in that chain are numbered in sequence substituents are numbered at their point of attachment Compound name is one word (German style) Complex substituents are named as compounds would

be


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Names of Small Hydrocarbons

No. of Carbons Formula Name (CnH2n+2)

1 Methane CH4

2 Ethane C2H6

3 Propane C3H8

4 Butane C4H10

5 Pentane C5H12

6 Hexane C6H14

7 Heptane C7H16

8 Octane C8H18

9 Nonane C9H20

10 Decane C10H22


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Nomenclature of Branched-Chain Alkanes (IUPAC) Locate the longest continuous chain of carbons; this is the parent chain and

determines the parent name.

Number the longest chain beginning with the end of the chain nearer the substituent Designate the location of the substituent

When two or more substituents are present, give each substituent a number corresponding to its location on the longest chain

Substituents are listed alphabetically


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When two or more substituents are identical, use the prefixes di-, tri-, tetra- etc.

Commas are used to separate numbers from each other The prefixes are used in alphabetical prioritization

When two chains of equal length compete to be parent, choose the chain

with the greatest number of substituents

When branching first occurs at an equal distance from either end of the parent chain, choose the name that gives the lower number at the first point of difference


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3.3 Alkyl Groups

Alkyl group – remove one H from an alkane (a part of a structure)

Naming: replace -ane ending of alkane with -yl ending CH3 is “methyl” (from methane)

CH2CH3 is “ethyl” from ethane

See Table 3.4 for a list


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Physical Properties

Boiling points and melting points increase as size of alkane increases

Forces between molecules (temporary dipoles, dispersion) are weak


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3.6 Cycloalkanes

Cycloalkanes are alkanes that have carbon atoms that form a ring (called alicyclic compounds)

Simple cycloalkanes rings of CH2 units, (CH2)n, or CnH2n

Structure is shown as a regular polygon with the number of vertices equal to the number of C’s (a projection of the actual structure)

cyclopropanecyclohexanecyclopentane

cyclobutane


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Complex Cycloalkanes

Naturally occurring materials contain cycloalkane structures Examples: chrysanthemic acid (cyclopropane),

prostaglandins (cyclopentane), steroids (cyclohexanes and cyclopentane)

6. Alkenes: Structure and Reactivity


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Alkene - Hydrocarbon With Carbon-Carbon Double Bond Also called an olefin but alkene is better Includes many naturally occurring materials

Flavors, fragrances, vitamins


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6.3 Naming of Alkenes Find longest continuous carbon chain containing the double

bond and name the compound using the suffix “ene” Number carbons in chain so that double bond carbons have

lowest possible numbers Rings have “cyclo” prefix


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Names of Small Alkene

No. of Carbons Formula Name (CnH2n)

1

2 Ethene C2H4

3 Propene C3H6

4 Butene C4H8

5 Pentene C5H10

6 Hexene C6H12

7 Heptene C7H14

8 Octene C8H16

9 Nonene C9H18

10 Decene C10H20


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Many Alkenes Are Known by Common Names Ethylene = ethene Propylene = propene Isobutylene = 2-

methylpropene Isoprene = 2-methyl-1,3-

butadiene


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6.3 Degree of Unsaturation

Relates molecular formula to possible structures Degree of unsaturation: number of multiple bonds or rings Formula for saturated a acyclic compound is CnH2n+2

Each ring or multiple bond replaces 2 H's


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Example: C6H10

Saturated is C6H14

Therefore 4 H's are not present

This has two degrees of unsaturation Two double bonds? or triple bond? or two rings or ring and double

bond

H3CC

CC

CCH3

H H

H H

H H

H H


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Degree of Unsaturation With Other Elements Organohalogens (X: F, Cl, Br, I)

Halogen replaces hydrogen C4H6Br2 and C4H8 have one degree of unsaturation Oxygen atoms - if connected by single bonds

These don't affect the total count of H's


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Compounds with the same degree of unsaturation can have many things in common and still be very different

Degree of Unsaturation and Variation


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If C-N Bonds Are Present

Nitrogen has three bonds So if it connects where H was, it adds a connection

point Subtract one H for equivalent degree of unsaturation in

hydrocarbon


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Count pairs of H's below CnH2n+2 Add number of halogens to number of H's (X equivalent to

H) Don't count oxygens (oxygen links H) Subtract N's - they have two connections

Summary - Degree of Unsaturation


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6.4 Cis-Trans Isomerism in Alkenes

The presence of a carbon-carbon double can create two possible structures cis isomer - two similar

groups on same side of the double bond

trans isomer similar groups on opposite sides

Each carbon must have two different groups for these isomers to occur


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Cis, Trans Isomers Require That End Groups Must Differ in Pairs

180°rotation superposes Bottom pair cannot be

superposed without breaking C=C

X


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6.6 Sequence Rules: The E,Z Designation Neither compound is

clearly “cis” or “trans” Substituents on C1 are

different than those on C2

We need to define “similarity” in a precise way to distinguish the two stereoisomers

Cis, trans nomenclature only works for disubstituted double bonds


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Develop a System for Comparison of Priority of Substituents

Assume a valuation system If Br has a higher

“value” than Cl If CH3 is higher than H

Then, in A, the higher value groups are on opposite sides

In B, they are on the same side Requires a universally

accepted “valuation”


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E,Z Stereochemical Nomenclature Priority rules of Cahn,

Ingold, and Prelog The Cahn-Ingold-Prelog

convention is used to assign the groups of highest priority on each carbon

Compare where higher priority group is with respect to bond and designate as prefix

E -entgegen, opposite sides

Z - zusammen, together on the same side


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Ranking Priorities: Cahn-Ingold-Prelog Rules Must rank atoms that are connected at comparison point Higher atomic number gets higher priority

Br > Cl > O > N > C > H

In this case,The higher priority groups are opposite:(E )-2-bromo-2-chloro-propene


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If atomic numbers are the same, compare at next connection point at same distance

Compare until something has higher atomic number Do not combine – always compare

Extended Comparison


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Substituent is drawn with connections shown and no double or triple bonds

Added atoms are valued with 0 ligands themselves

Dealing With Multiple Bonds

mcmurry organic chemistry 6th edition chapter 3 (c) 2003 2 representations of structural formulas...

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