Chapter 3 - Organic compounds: Alkanes and their stereochemistry

What is a functional group?

A group of atoms within a molecule that has a characteristic chemical behavior.

Alkene

Alkyne

Arene

Halide

Alcohol

Ether

Ending: ether

Monophosphate

Ending: phosphate

OPO₃^2-

Diphosphate

Ending: diphosphate

OP₂O₆^3-

Amine

Ending: -amine

NH₂

Imine

Ending: none

NH

Nitrile

Ending: -nitrile

N

Thiol

Ending: -thiol

SH

Sulfide

Ending: sulfide

Disulfide

Ending: disulfide

SS

Sulfoxide

ending= -sulfoxide

SO^-

Sulfoxide can also exist with double bond to O

Aldehyde

ending -al

COH

Ketone

Ending: one

C=O

Carboxylic acid

Ending: -oic acid

COOH

Ester

Ending = -oate

COOCH₃

Thioester

Ending: -thioate

COSCH₃

Amide

Ending: -amide

CONH₂

Acid chloride

Ending: -oyl chloride

COCl

Carboxylic acid anhydride

Ending: -oic anyhydrided

COOCOCH₃

Saturated hydrocarbonds

Hydrocarbon: consists of carbon and hydrogen only

Saturated: They have only C-C and C-H single bonds and thus contain the maximum possible number of hydrogens per carbon.

General formula alkane

C_nH_2n+2

Straight-chain alkanes vs. branched chain alkanes

Straight chain: All carbons are connected in one row.

Branched: Where the carbons branch off.

Isomers

Same formula, different structures.

Isomers are compounds that have the same numbers and kinds of atoms but differ in the way the atoms are arranged.

Consists of constitutional isomers and stereoisomers.

Constitutional isomers.

Are isomers whose atoms are connected differently

How can a normal straight chain be represented?

putting: n- before the formula

e.g.: n-C₄H₁₀

Number of carbons

1: Meth

2: Eth

3: Prop

4: But

5: Pent

6: Hex

7: Hept

8: Oct

9: Non

10: Dec

Alkyl naming

Number (e.g. Meth) + yl

primary, secondary, tertiary and quaternary carbon

Primary carbon: is bonded to one other carbon.

Secondary carbon: is bonded to two other carbons

etc.

e.g. a tertiary alcohol = has an alcohol functional group bonded to a carbon atom that is itself bonded to three other carbons.

Primary, secondary, … hydrogen

Primary hydrogen atoms are attached to primary carbons (RCH3), secondary hydrogens are attached to secondary carbons (R2CH2), and tertiary hydrogens are attached to tertiary carbons (R3CH)

(no quaternary hydrogen because then the carbon would have an expanded octet)

Steps of IUPAC

1. Find the longest continuous chain of carbon atoms in the molecule and use the name of that chain as the parent name. (if two chains with equal length are present choose the one that has the most branch points)

2. Number each carbon in the parent chain. Try to have the branches at low numbers

3. Identify and number the substituents

4. Write the name as a single word. Use hyphens to separate different prefixes. Cite prefixes in alphabetical order.

Propyl vs isopropyl

"iso-" signifies a specific type of branching pattern where the last two carbons of the chain are part of a terminal isopropyl group

Properties of alkanes

Have little affinity and are inert.

Reaction of alkane with oxygen

• Reaction with oxygen occurs during combustion in an engine or furnace when an alkane is used as fuel.

• Carbon dioxide and water are formed as products, and a large amount of heat is released.

Reaction of alkane with Cl₂

• Occurs when a mixture of the two is irradiated with ultraviolet light .

• Depending on the time allowed and the relative amounts of the two reactants, a sequential substitution of the alkane hydrogen atoms by chlorine occurs.

• This leads to a mixture of chlorinated products

Relation number of carbons to boiling/melting point

Higher number of carbons = higher melting/boiling point

Effect of increased branching on boiling point

Another effect seen in alkanes is that increased branching lowers an alkane’s boiling point.

• e.g. pentane has no branches and boils at 46.1 C

• Isopentane (2-methylbutane) has one branch and boils at 27.85 C

Stereochemistry

The branch of chemistry concerned with the three-dimensional aspects of molecules.

Conformation

The different arrangements of atoms that result from bond rotation are called conformations

Conformers

• Another word for conformational isomers

• Molecules that have different arrangements

Conformational isomers vs. constitutional isomers

Conformational isomer: Same molecular formula, same connectivity, but differ by rotation around single bonds

Constitutional isomer: Same molecular formula, but differ in connectivity (bonding sequence) of atoms

Newman projection vs. Sawhorse representation

Sawhorse: views carbon-carbon bond from an oblique angle and indicates spatial orientation by showing all C-H bonds.

Newman: Views the carbon-carbon bond directly end-on and represents the two carbon atoms by a circle.

Staggered conformation

A conformation where substituents on adjacent carbons are positioned as far apart as possible, minimizing steric hindrance.

• Seen as the most stable conformation due to lower torsional strain.

• Maximum distance

• Lowest energy

Eclipsed conformation

A conformation where substituents on adjacent carbons are aligned directly with each other, causing increased steric hindrance.

• Less stable due to increased repulsion and torsional strain between groups.

• Minimum distance

• Highest energy

What is the lowest-energy arrangement called?

The anti conformation, where the two methyl groups are as far apart as possible. (180 degrees)

Gauche conformation

When an energy minimum is reached at the staggered conformation where they methyl group are 60 degrees apart.

• Has no eclipsing interaction

• This energy difference occurs because the hydrogen atoms of the methyl groups are near one another in the gauche conformation, resulting in what is called steric strain.

How do molecules shift from eclipsed to staggered conformation?

• Through rotation around a single sigma bond.

• During rotation the molecule moves between energy states. It has to overcome a small energy barrier to go from one conformation to another.

  • In eclipsed conformation, this is a high-energy state because the electron clouds of substituent groups on adjacent carbons align directly with each other, causing torsional strain from steric repulsion.

  • In staggered conformation this is a low energy state where substituent groups are as far apart as possible, minimizing repulsive forces and making it more stable.

Steric strain

The repulsive interaction that occurs when atoms are forced closer together than their atomic radii alow.

Three types of cycloalkanes

1. Chrysanthemic acid

2. Prostaglandins

3. Steroids

Chrysanthemic acid

• Active insecticidal

• Constituents of chrysanthemum flowers

Prostaglandins

• potent hormones

• control physiological functions in humans

Steroids

• naturally occurring hormones in plants and animals

Cis-trans isomerism in cycloalkanes

• Two faces

• Isomerism possible in substituted cycloalkanes.

• Substituted cycloalkanes have substituents. Instead of just C and H.

Acyl group