TOPIC 1: ORGANIC COMPOUNDS: ALKANES AND CYCLOALKANES According to Chemical Abstracts, there are more than 18 million known organic compounds, each of which has its own physical properties, such as melting point and boiling point, as well as its own reactivity.

Chemists have learned that organic compounds can be classified into families according to their structural features and that members of a given family often have similar chemical behavior.

Instead of 18 million compounds with random reactivity, there are a few dozen families of organic compounds whose chemistry is reasonably predictable.

Throughout this course, we will study the chemistry of specific families.

I. FUNCTIONAL GROUPS

FUNCTIONAL GROUP:- Group of atoms within a molecule that has a characteristic chemical behavior

- Behaves nearly the same way in every molecule

Caffeine

Prozac

Acetominophen Vanillin

Taxol, isolated from the bark of the Pacific Yew tree, Taxus brevifolia, is an anticancer drug, which was first studied in the successful treatment of ovarian cancer.

In 1969, 1,200 kg of tree bark was processed to produce10.0 g of pure taxol. The molecule contains 14 functional groups.

Taxol

Since it would be impractical to harvest enough trees to obtain significant amounts of taxol, a semi-synthetic methodwas developed.

Needles are harvested from a related, low growing evergreen, Taxus baccata, which is grown inplantations similar to growing tea.

The needles are processed to produce large amounts of a synthetic precursor 10- deacetylbaccatin whose structure is shown below.

10- deacetylbaccatin- Synthetic precursor to taxol- More readily available

II. ALKANES AND ALKANE ISOMERS* You saw in Background Unit One that carbon-carbon singlebonds result from head-on overlap of carbon sp3 hybridized orbitals.One of the unique properties of carbon is its ability to bond to multiple carbon atoms results in long chains of atoms and the formation of complex molecules.

ALKANES:- contain single carbon to carbon bonds- hydrocarbons: contain only C & H atoms- saturated: contain the max. number of H atoms per C atom- have the general formula: CnH2n+2 (n = any integer)- nonpolar

STRAIGHT-CHAIN or NORMAL ALKANES:- carbons atoms are connected in a row- Ex.

BRANCHED-CHAIN ALKANES:- carbons atoms branch from the main chain- Ex.

CONSTITUTIONAL ISOMERS:- contain the same number & types of atoms but differ in the way the atoms are arranged- have different chemical and physical properties

Different carbon skeletons: C4H10

Different functional groups: C2H16O

Different position of functional groups: C3H9N

OH O

alcohol etherNH2

NH2

Isomer Possibilities:

* Also remember that carbon- carbon single bonds can rotate. What may at first seem like two different molecules may really just be the same molecule represented in two different ways.

1

2

3

4

5 1

2

34

5

rotate around C-4 These two molecules are the same; they are not isomers

III. NAMING ALKANES* In earlier times when relatively few pure organic moleculeswere known, new compounds were named at the whim of theirdiscoverer. Thus, urea (CH4N2O) is a crystalline substance isolated

from urine; morphine (C17H19NO3) is a painkiller named after

Morpheus, the Greek god of dreams; and barbituric acid is a tran-quilizing agent named by its discoverer in honor of his friendBarbara.

As the number of known organic compounds slowly grew, so did theneed for a systematic way of naming them. Although many “common” naming for organic compounds still exist, the system of naming we used is devised by the International Union of Pure and Applied Chemistry (IUPAC, usually spoken as “eye-you-pac).

A chemical name in the IUPAC system has three parts:

prefix parent suffix

Where are the substituents?

How manycarbons?

What is themain functionalgroup present?

Naming Alkanes

The names of alkanes are derived from a set of prefixes, used to indicate the number of carbons present

Prefix Formula Prefix Formula

C1 C6

C2 C7

C3 C8

C4 C9

C5 C10

meth-

eth-

prop-

but-

pent-

hex-

hept-

oct-

non-

dec-

The names of alkanes end in -ANE

YOU MUST KNOW THESE!

Practice: Name the following alkanes

IV. PROPERTIES AND REACTIONS OF ALKANES

* Alkanes are sometimes referred to as “paraffins”- a word derived from the Latin parum affinis, meaning “slight affinity.” This term aptly describes their behavior because alkanes show little affinity for other some substances and are, for the most part, chemically unreactive.

PROPERTY 1: ALKANES SHOW REGULAR INCREASES IN BOILING & MELTING POINTS AS THEIR MOLECULAR WEIGHT INCREASES

- Due to van der Waals forces: weak intermolecular forces that operate over small distances & occur because the electron distribution in the molecules are non-uniform at any given instant & create temporary dipoles

Bigger molecules

= stronger temporary dipoles

= more energy (higher temps.) needed to break the van der Waals forces

PROPERTY 2: INCREASED BRANCHING LOWERS AN ALKANES BOILING POINT

- Branched-chain alkanes are more spherical, have smaller surface areas, and therefore have weaker van der Waals forces

- Weaker van der Waals forces = lower boiling point

REACTION 1: COMBUSTION

- When hydrocarbons burn in the presence of O2, the products are H2O and CO2

- A lot of heat is produced- Ex. CH4 + O2 CO2 + H2O

REACTION 2: RADICAL HALOGENATION

- Mixture of alkane and X2 is irradiated w/ ultraviolet light

(denoted as hυ) forming the X• radical (odd electron species)

- Products contain a variety of halogen substituted products- Cons of reaction: difficult to control the exact spot of

halogenation (see example below) & mixtures of products form

h Cl2

ClCl Cl Cl

ClCl etc...

V. CYCLOALKANES CYCLOALKANES:

- also called cyclic compounds- consist of rings of – CH2 – units- have the general formula: CnH2n

The general class of compounds known as steroids contain four rings joined together- three of them six-membered rings and one of them a five membered ring.

Cortisol, also known as hydrocortisone, an over the counter medicine used to treat skin irritations.

Cholesterol is essential for all animal life. Each cell synthesizes it from simpler molecules, a complex 37-step process.

VI. CIS-TRANS ISOMERISM IN CYCLOALKANES

* In many ways, cycloalkanes behave similar to their open- chain counterparts. One difference is that cycloalkanes are less flexible. In smaller rings (C3 – C6), rotation about the C – C bond is impossible. This leads to the ring having a “top” and a “bottom” side.

CH3

H

H

CH3H

H

CH3

H

CH3

HH

H

cannot be converted to

STEREOISOMERS:- atoms are connected in the same order but differ in their

spatial orientation

CIS –TRANS ISOMERS:- a special type of stereoisomers- cis: means groups are on the same side- trans: means groups are on the opposite side

CH3

H

H

CH3H

H

CH3

H

CH3

HH

Hcis-1,2-dimethylcyclopropane trans-1,2-dimethylcyclopropane

A. 1,4- dimethylcyclohexaneB. 1-methyl-3-propylcyclopentaneC. 3-cyclobutylpentaneD. 1-bromo-4-ethylcyclodecaneE. 1-chloro-3-ethyl-2-methylcyclopentaneF. 1-bromo-3-ethyl-5-propylcyclohexaneG. 1-fluoro-2-methylcyclobutaneH. 1-ethyl-4-methylcycloheptaneI. 1,1,4-trimethylcyclohexaneJ. 3-cyclopentylheptaneK. 1,3-dimethylcyclopentaneL. 1-ethyl-2,2,6,6-tetramethylcyclohexane

A. 3-ethyl-4-methylhexane

B. 4-ethyl-5,6-dimethyl-7-propyldecane

C. 3,4,5-triethyl-6-methyloctane

D. 3,4-diethyl-2-methylhexane

E. 1,2-diethyl-3-methylcyclohexane

F. 1,4-dibromo-2-fluorocyclopentane

G. 1-cyclopropyl-2-propylcyclopentane

H. 2-cyclopentyl-6-methyloctane

I. 5-ethyl-3-methyl-6-propylnonane