chapter 3 alkanes and their stereochemistry-1-1.ppt

3. Organic Compounds: Alkanes and Cycloalkanes

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Classification of Organic Compounds Organic compounds are classified into different types based

on the kinds of functional groups present in its structure A functional group is an atom or groups of atoms that

determine the types of chemical reactions that the compound participates in.

For example, the double bonds in simple and complex alkenes react with bromine in the same way

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

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Types of Functional Groups: Multiple Carbon–Carbon Bonds Alkenes have a

C=C double bond Alkynes have a

CC triple bond In Arenes or

aromatic hydrocarbons, the functional group is the benzene ring


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Functional Groups with Carbon Singly Bonded to an Electronegative Atom Alkyl halide: C bonded to halogen (C-X) Alcohol: C (part of aliphatic group) bonded O of a

hydroxyl group (C-OH) Phenol: C (part of aromatic group) bonded O of a

hydroxyl group (C-OH) Ether: Two C’s bonded to the same O (C-O-C) Amine: C bonded to N (C-N) Thiol: C bonded to SH group (C-SH) Thioether: Two C’s bonded to same S (C-S-C) Bonds are polar, with partial positive charge on C (+)

and partial negative charge () on electronegative atom


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Groups with a Carbon–Oxygen Double Bond (Carbonyl Groups)

Aldehyde: one hydrogen bonded to C=O Ketone: two C’s bonded to the C=O Carboxylic acid: OH bonded to the C=O Ester: C-O bonded to the C=O Amide: C-N bonded to the C=O Acid chloride: Cl bonded to the C=O Carbonyl C has partial positive charge (+) Carbonyl O has partial negative charge (-).

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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 general molecular formula of an alkane with n C atoms is CnH2n+2

When n 4, more than one structural formula can be drawn for the same molecular formula, CnH2n+2

Compounds with the same molecular formula but with different structural formulas are called structural isomers or constitutional isomers.

Structural isomers have different physical properties (Boiling points, melting points, densities etc.)

Alkanes are called saturated hydrocarbons because they are saturated with hydrogen (no more can be added

Alkanes, Alkenes, Alkynes, Cycloalkanes, Cycloalkenes and Cycloalkynes are also classified as aliphatic hydrocarbons

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Alkane Isomers CH4 = methane, C2H6 = ethane, C3H8= propane Isomers of alkanes are of 2 types: straight chain or normal

alkanes and branched chain alkanes. Straight-chain or normal alkanes have all n C atoms attached

in a single connected chain. That is, no C atom is attached to more than 2 other C atoms. For any alkane with n 4, there is only one straight chain isomer.

Branched-chain alkanes have one or more C atoms connected to 3 or 4 other C atoms. C4H10 has one branched isomer. For all alkanes with n > 4, there are more than one branched isomers.


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Examples of Constitutional Isomers Compounds other than alkanes can be constitutional isomers of

one another They must have the same molecular formula to be isomers


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Condensed Structures of Alkanes We can represent an alkane in a brief form or in many types of

extended form A condensed structure does not show bonds but lists atoms,

such as CH3CH2CH3 (propane) CH3C(CH3)2CH3 (IUPAC: 2,2-dimethylpropane; common:

neopentane)


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Names of Small HydrocarbonsNo. 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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3.3 Alkyl Groups Alkyl group – remove one H from an alkane

(a part of a structure) General abbreviation “R” (for Radical, an

incomplete species or the “rest” of the molecule)

Name: 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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Types of Alkyl groups Classified by the connection site (See Figure 3.3)

a carbon at the end of a chain (primary alkyl group) a carbon in the middle of a chain (secondary alkyl

group) a carbon with three carbons attached to it (tertiary alkyl

group)

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3.4 Naming Branched Alkanes Find the longest continuous chain of C atoms. This is the parent

chain. Name the parent chain Number the C atoms in the parent chain starting from the end

closest to a substituent Substituents are atoms (Cl, Br, I) or groups of atoms (examples:

alkyl groups, OH, carbonyl group, amino group) that have replaced H’s

Specify the number(s) of the C atom(s) to which the substituent(s) is(are) attached to and the name(s). Letters and numbers are separated by hyphens

List the substituents in alphabetical order before the parent name. When there are more than 1 substituent of the same type, use

prefixes di, tri, tetra etc. to specify the number of each type. The location of each substituent (even when they are identical and attached to the same C) should be specified.


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•If two different longest chains are possible, choose the one with the more substituents as the parent chain•If two different substituents are equidistant from either end, chose to number from that end that is closer to the substituent that is alphabetically first.


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Drawing structural isomers for alkanes with n≥4 C atoms: Start with parent straight chain with n≥3. Replacing any H attached to a secondary C with a methyl group will result in a branched isomer with n+1 C atoms. Replacing any H attached to either of the two primary C atoms of a straight chain alkane will only yield the corresponding straight chain alkane with n+1 C atoms. When drawing isomers, replacing any of the H on symmetrically positioned C atoms with the same alkyl group will yield the same isomer. Verify by naming your isomers. If the names of two isomers are the same, they are identical.

Drawing Branched Isomers

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3.5 Properties of Alkanes Alkanes undergo substitution reactions and combustion

reactions (alkane + O2 → CO2(g) + H2O) In substitution reactions, H’s are replaced by other atoms. For example,alkanes react with Cl2 in the presence of light

to replace H’s with Cl’s. The boiling points & melting points of alkanes increase

with increasing chain length. Among the structural isomers of an alkane, the less

branched the alkane, higher its boiling point, melting point.

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3.6 Cycloalkanes Cycloalkanes are alkanes that have carbon atoms attached to form a

ring In simple cycloalkanes, all n C atoms are attached to form a ring. The

general molecular formula of a cycloalkane with n C atoms is CnH2n The condensed structural formula of a simple cycloalkane with n C

atoms is a regular polygon with n vertices The boiling points of cycloalkanes also increases with increase in ring

size. Cycloalkanes exhibit structural isomerism. A cycloalkane has the

same general formula as an alkene with the same number of C atoms and 1 double bond.

In addition, any cycloalkane with more than 3 C atoms will also have cyclic structural isomers

cyclopropanecyclohexane

cyclopentanecyclobutane


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

(cyclopentane), steroids (cyclohexanes and cyclopentane)


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3.7 Naming SubstitutedCycloalkanes Count the number of carbon atoms in the ring and the number of C atoms in the

largest substituent chain. If the number of carbon atoms in the ring is equal to or greater than the number in the substituent chain, the compound is named as an alkyl-substituted cycloalkane. Otherwise, if the number of C atoms is greater in the alkyl chain, the compound is named as a cycloalkyl-substituted alkane

There is no need to number the ring if there is only 1 substituent. When there are more than 1 substituent, the ring has to be numbered. For an

alkyl- or halo-substituted cycloalkane, start at a point of attachment as C1 and number the substituents on the ring so that the second substituent has as low a number as possible.

Number the substituents and write the name

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3.8 Cis-Trans Isomerism in Cycloalkanes Stereoisomers have the same molecular formula, the same

connectivity but different 3-D orientations of substituents. The rotation about C-C bonds in cycloalkanes is limited which results

in stereo isomerism when there are 2 or more substituents. Rings have two “faces” and the relative positions of two different

substituents will result in different stereoisomers. In disubstituted cycloalkanes, the isomer with both substituents on

the same side is called the cis isomer and the isomer with substituents on opposite sides is called the trans isomer.

For example, consider the 2 stereoisomers of 1,2-dimethyl-cyclopropane . Cis-1,2-dimethylcyclopropane has both methyl substituents on the same side. Trans-1,2-dimethycyclopropane has the methyls on opposite sides of the ring


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Structural Isomers & Stereoisomers


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Naming Bicycloalkanes & Spiroalkanes

1. A bicycloalkane consists of 2 rings that share 2 C atoms. The two shared C atoms are called ‘bridgehead C’s’. There are 3 paths or ‘bridges’ that connect these two bridgehead C’s.

2. The parent name of a bicycloalkane is that of an unbranched alkane having the same number of carbons with a prefix bicyclo.

3. Begin numbering at one of the two "bridgehead" carbons. Proceed around the longest bridge to the 2nd bridgehead, then around the next longest bridge and then finally around the shortest bridge.

4. If the one or more substituents are present, start numbering from the bridgehead which results in the first substituent on the larger bridge having a smaller number.

5. The length of a ‘bridge’ = number of C in that bridge between the bridgehead C’s and not including the bridgehead C’s.

6. The lengths of the bridges are specified within square brackets between the word bicyclo and the parent name. The lengths are written in descending order and separated by periods. For example, bicyclo[4.2.1]nonane has 9 C atoms two of which are bridgehead C’s and therefore common to all 3 bridges. The 3 bridges have 4 C, 2C and 1 C respectively.


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Naming Spiroalkanes1. A spiroalkane consists of 2 rings that share 1 C atom. 2. The parent name of a spiroalkane is that of an unbranched alkane having the same

number of carbons with a prefix spiro. 3. Count the C’s in the smaller ring and the C’s in the larger ring to determine the bridge

lengths. Do not include the shared C in determining the bridge lengths. 4. If substituents are present on the ring, numbering of the rings starts in the small

ring on the C next to the shared C, passes round this ring and through the shared C into the larger ring. The numbering goes round the two rings in the direction which produces the lowest numbers in each ring..

5. The lengths of the bridges are specified within square brackets between the word spiro and the parent name.

6. The lengths are written in ascending order and separated by periods. For example, spiro[3.4]octane has 8 C atoms one of which is a bridgehead C and therefore common to both rings. The 2 bridges have 3 Cand 4 C respectively.

Check the following site for problems: http://www.chem.ucalgary.ca/courses/351/orgnom/alkanes/alkanes-08.html

chapter 3 alkanes and their stereochemistry-1-1.ppt

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