t opic 1: o rganic c ompounds : a lkanes and c ycloalkanes according to chemical abstracts, there...
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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.
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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
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Caffeine
Prozac
Acetominophen Vanillin
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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
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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
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II. ALKANES AND ALKANE ISOMERS
* You saw in Background Unit One that carbon-carbon single
bonds 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.
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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.
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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 ether
NH2
NH2
Isomer Possibilities:
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* 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
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III. NAMING ALKANES* In earlier times when relatively few pure organic molecules
were known, new compounds were named at the whim of their
discoverer. 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 friend
Barbara.
As the number of known organic compounds slowly grew, so did the
need 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).
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A chemical name in the IUPAC system has three parts:
prefix parent suffix
Where are the substituents?
How manycarbons?
What is themain functionalgroup present?
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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!
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Practice: Name the following alkanes
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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.
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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
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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
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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
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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...
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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.
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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
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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
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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
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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