stereochemistry

Organic Functional groupand Stereochemistry

Families of Organic Compounds
Organic compounds can be grouped into families by their common structural features. We shall survey the nature of the compounds in a tour of the families in this course

Functional Groups

Functional group - collection of atoms at a site within a molecule with a common bonding pattern The group reacts in a typical way, generally independent of the rest of the molecule For example, the double bonds in simple and complex alkenes react with bromine in the same way.


Types of Functional Groups: Multiple Carbon–Carbon Bonds Alkanes have a C-C single bond Alkenes have a C-C double bond. Alkynes have a C-C triple bond


Groups with a Carbon–Oxygen Double Bond (Carbonyl Groups Aldehyde: one hydrogen bonded to C=OKetone: two C’s bonded to the C=OCarboxylic acid: OH bonded to the C=OEster: C-O bonded to the C=OAmide: C-N bonded to the C=OAmine: C bonded to N (C-N)Carbonyl C has partial positive charge (+)Carbonyl O has partial negative charge (-).

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

Alkanes and Alkane Isomers
Alkanes: Compounds with C-C single bonds and C-H bonds only (no functional groups)Connecting carbons can lead to large or small moleculesThe formula for an alkane with no rings in it must be CnH2n+2 where the number of C’s is nAlkanes are saturated with hydrogen (no more can be addedThey are also called aliphatic compounds

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

Names of Larger Hydrocarbons

No. of Carbons
Formula Name
(CnH2n+2)
11
Undecane
C12H26
12
Dodecane
C12H26
13
Tridecane
C13H28
14
Tetradecane
C14H30
15
Pentadecane
C15H32
16
Hexadecane
C16H34
17
Heptadecane
C17H36
18
Octadecane
C18H38
19
Nonadecane
C19H40
20
Isocane
C20H42

Different Ways to Write Butane

Isomers
Isomers: are molecules with the same molecular formula, but different arrangements of atoms. There are different types of isomers, shown by the diagram on the right.

Alkane Isomers

CH4 = methane, C2H6 = ethane, C3H8= propane The molecular formula of an alkane with more than three carbons can give more than one structural isomer C4H10
C5H12

Constitutional Isomers

Isomers that differ in how their atoms are arranged in chains are called constitutional isomers Compounds other than alkanes can be constitutional isomers of one another They must have the same molecular formula to be isomers

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

Types of Alkyl groups

Classified by the connection site 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)

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 See specific examples in text

Nomenclature of Alkanes

Determine the number of carbons in the longest continuous chain
*

2. Number the chain so that the substituent gets the lowest number

*

Number the substituents to yield the lowest possible number in the number of the compound

substituents are listed in alphabetical order
*

Cycloalkanes

Cycloalkanes are alkanes that have carbon atoms that form a ring (called alicyclic compounds)Simple cycloalkanes rings of CH2 units, (CH2)n, or CnH2nStructure is shown as a regular polygon with the number of vertices equal to the number of C’s (a projection of the actual structure) cyclopropane
cyclohexane
cyclopentane
cyclobutane

Naming Cycloalkanes

Count the number of carbon atoms in the ring and the number 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, the compound is named as an alkyl-substituted cycloalkane 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 See text for more details and examples

Cis-Trans Isomerism in Cycloalkanes

Rotation about C-C bonds in cycloalkanes is limited by the ring structureRings have two “faces” and substituents are labeled as to their relative facial positionsThere are two different 1,2-dimethyl-cyclopropane isomers, one with the two methyls on the same side (cis) of the ring and one with the methyls on opposite sides (trans)

Amine groups NH3

Primary amines: In primary amines, only one of the hydrogen atoms in the ammonia molecule has been replaced. That means that the formula of the primary amine will be RNH2 where "R" is an alkyl group.


Secondary amines: In a secondary amine, two of the hydrogens in an ammonia molecule have been replaced by hydrocarbon groups. At this level, you are only likely to come across simple ones where both of the hydrocarbon groups are alkyl groups and both are the same.


Tertiary amines In a tertiary amine, all of the hydrogens in an ammonia molecule have been replaced by hydrocarbon groups. Again, you are only likely to come across simple ones where all three of the hydrocarbon groups are alkyl groups and all three are the same.

Stereoisomers

is the study of molecules on space how atoms or groups in a molecule arranged in space relative to each other it is that part of the science which deals with structure in three dimensions

Stereoisomerism Optical active isomers. Geometrical isomers. Conformational isomers

* 3.2: Conformational Analysis of Butane

Staggered: anti
Staggered: gauche
3 KJ/mol

Geometric isomerism

An isomerism resulting from rigidity in molecules and accures only in two class of compounds alkenes and cyclic compounds
σ – bond free rotation ( flexibility )  - bond C = C no free rotation ( rigidity ) σ – bond restricted rotation ( semi-fexable )

Optical isomerism

Optical isomerism occurs when substances have the same molecular and structural formulae, but one cannot be superimposed on the other. Put simply, they are mirror images of each other

(+)-lactic acid

(–)-lactic acid The dashed lines show bonds going into the screen; the wedges show bonds coming out of the screen.

Triglycerides: Three Fatty Acids Dehydrated to One Glycerol Steroids Proteins(amino acids ) DNA