Introduction
What
do people think organic is? A survey was conducted to ask participants
about what they thought organic meant. Most of the replies referred to
organic foods, while only a few science majors said carbon compounds.
Organic chemistry is the study of the structure, properties, and
reactions of carbon compounds. Even though organic chemistry focuses on
carbon, many organic compounds can contain hydrogen, nitrogen, oxygen,
phosphorous or other elements. Carbon molecules have many different
functions; they are commonly used in medicine, food, paints, and
gasoline.
Carbon-containing Compounds
Organic
chemistry focuses on the structure, properties, and applications of
various carbon-containing molecules that make up important biological
molecules such as proteins, enzymes, carbohydrates, lipids, nucleic
acids, and vitamins. A number of organic compounds are found in nature
such as cotton, wool, and natrual petroleum while others are purely
man-made. Industries have been able to synthesize many organic compounds
such as plastics, photographic film, synthethic fabrics, and
pharmeceutical drugs using applications of organic chemistry.
Organic Foods
Many
people think of organic food as the healthier alternative because they
associate organic foods with no pesticides, no chemicals and are not
genetically modified. That may be true because the United States
Departments of Agriculture has specific standards farmers have to
follow. The questions are what is the difference between organic and
non-organic produce? Are organic foods more nutritional? There have not
been any real evidences about whether organic foods are more nutritional
than non-organic foods. There are not as many differences between
organic and non-organic as one would think, people claim that they can
taste the difference between the two, but taste is solely based on
personal preferences. The only differences between organic and
non-organic foods are the way the food were grown, they do not look as
"perfect" as non-organic foods and they spoil faster.
The
definition of organic is: "having properties or characteristics of
living organisms" (21). It is true that organic foods were from living
beings, but the fact that people are associating organic foods as:
pesticide free, hormone free, and other natural processes, is not the
correct perspective. "Organic" just means things from living organisms,
including: organs, hair, fruits, and even seeds.
A Brief History of Organic Chemistry
The
word organic comes from the term "organism", thus explaining the focus
of organisms in organic chemistry. In 1770, a Swedish chemist named
Torbern Bergman made an important distinction between organic and
inorganic substances. In 1807, Jöns Jacob Berzelius was the first person
to call components from biological sources "Organic Chemistry". Before
the modernization of organic chemistry in the early 19th century,
scientists had discovered major distinctions between extracts of living
and non-living things. A theory called the vital force theory, or
vitalism, stated that there was a force in organic molecules that
inorganic molecules do not have. In 1828, Friedrich Wöhler converted
ammonium cyanate (inorganic compound) into urea. This event is
considered the start of organic chemistry. Wöhler proved the vitalism
theory to be incorrect because it was believed that it was not possible
to make organic compounds out of inorganic compounds.
This animation displays how Ammonia and Cyanate combined to form Urea.
Why learn Organic Chemistry?
Organic
chemistry is everywhere as every living organism contains organic
compounds. Organic chemistry has many applications in medicine, biology,
technology, industrial production, and business. Other applications of
organic chemistry include making plastics, gasoline, detergent and
plenty of other industrial products. Organic chemistry is an important
aspect of our lives.
Why is Carbon so special?
Carbon has an atomic number of 6; it is a second row element in the p-block.
Carbon
is the sixth most abundant element in the universe (24). Carbon is able
to form strong polar and non-polar covalent bonds by sharing its
electrons to construct long chains and various structures. Some examples
of polar covalent bonds are: C-F, C-O, and C-N. Some examples of
non-polar covalent bonds are: C-C and C-H. Carbon can also attach with
many metallic and nonmetallic elements. When a carbon shares its
electrons with hydrogens, a hydrocarbon is formed which is considered to
be the simplest organic compound. A unique property of carbon is that
it can form many polymers in different structures like tubes, spheres,
rings and chains. Carbon likes to form four bonds, which allows it to
interact with a variety of molecules. The 4 bonds are essential for the
formation of stable molecules. Carbon can bond in many different ways
with many different elements.
How is Organic Different from Inorganic Chemistry?
Organic
chemistry is defined as chemistry of living things. Most compounds
discussed in organic chemistry are made up of carbon molecules. Many
people think that organic chemistry revolves around carbon. However,
that is not the case; there are some carbon molecules that are
inorganic, such as carbon dioxide (CO2). Organic chemistry not only
focuses on carbon but also on how electrons move in carbon compounds.
The most simple answer for how organic is different from inorganic is
that organic molecules are from living organisms while inorganic comes
from non-living resources.
Introduction to Organic Chemistry
Organic
chemistry is the chemistry of carbon; we know that carbon can also bond
with many different elements forming various compounds.
Before learning organic chemistry, a review of the following topics may be useful:
Molecular Representations
There
are a number of ways that a molecule or atom can be drawn. Some are
simpler than others, depending on the type of information that the image
is trying to convey. These representations are especially important in
organic chemistry when considering the connectivity of atoms, electron
distribution, formal charge, or bond type.
Skeletal Structure (Kekulé Structure)
In
this form of representation, atoms are placed on a plane and lines are
drawn between atoms to represent bonding electrons. Lone pairs are not
included in this representation.
Condensed Structure
A
simplified version of the bond-line structure that omits the lines.
When there are 2 or more of the same kinds of atoms attached to a
central atom, a subscript is used to indicate how many of these atoms
are attached.
Lewis Structure
In
this structure, valence electrons are represented as dots. This
structure shows us what atoms are bonded together, which electrons are
involved in bonding, lone pairs, and any formal charges.
Perspective Formula
In
this formula, bonds that are on the plane are drawn normally. Bonds
that protrude out of the plane towards the viewer are drawn as black
wedges. Bonds that go into the plane away from the viewer are drawn as
dashed wedges.
Naming Hydrocarbons
The simplest organic compounds are hydrocarbons. Hydrocarbons are compounds that consist of hydrogen and carbon atoms.
When naming hydrocarbons, the prefixes vary depending on the number of carbons in a compound, the prefixes are:
See Nomenclature for Organic Chemistry for more information on how to properly name organic molecules.
A trick to remember the prefixes of Met-, Eth-, Prop-, and But- is: Monkeys Eat Pink (or Plenty) Bananas.
The
simplest type of hydrocarbons are called alkanes which consist only of
single bonded atoms with a molecular formula of CnH2n+2 where n is equal
to the number of carbons. They are named using the prefixes above, and
by adding the suffix, "-ane."
Alkanes:
CH4 - Methane
C2H6 - Ethane
C3H8 - Propane
C4H10 - Butane
Other
kinds of hydrocarbons include: alkenes and alkynes. Alkenes are carbon
compounds with at least 1 double bond between 2 carbon atoms, with the
molecular formula CnH2n. They are named by adding the suffix, "-ene" to
the prefix based on the number of carbons.
Alkenes:
C2H4 - Ethene
C3H6 - Propene
C4H8 - Butene
Alkynes
are carbon compounds with one triple bond between two carbon atoms and
are expressed with the molecular formula, CnH2n-2. They are named by
adding the suffix, "-yne" to the prefix based on the number of carbons.
C2H2 - Ethyne
C3H4 - Propyne
C4H6 - Butyne
Naming Alkyl Compounds
Alkanes
are often present as alkyl substituents (alkyl groups) that attach to
other groups in larger organic molecules. Their molecular formulas have
one less hydrogen than a normal alkane. They are named by replacing the
normal alkane ending "-ane" with "-yl." The letter "R" commonly refers
to any alkyl functional group.
Alkyl Substituents:
CH3- methyl
CH3CH2- ethyl
CH3CH2CH2-propyl
CH3CH2CH2CH2- butyl
R- any alkyl group
Examples of compounds with alkyl groups:
CH3I: methyl iodide CH3CH2OH: ethyl alcohol
CH3CH2CH2NH2: propylamine R-O-R: an ether
CH3NH2: methylamine CH3CH2OCH3: ethyl methyl ether
CH3CH2CH2CH2Cl: butyl chloride R-NH2: an amine
Note that when there are two or more alkyl groups present, as in an ether, they must be stated in alphabetical order.
Structures of Organic Compounds
Alcohols:
consists
of a hydrocarbons chain that has a hydroxide in place of hydrogen
(anywhere in the compound). The common formula for an alcohol is R-OH,
where R is any hydrocarbon (residue group). Naming alcohols is not hard
either; for example propanol which comes from the propane hydrocarbon
therefore instead of having the –ane suffix you have –anol. Other
examples are ethanol, butanol, Alcohols have the ability of making acids
solutions. A common reaction of alcohols is with carboxylic acids to
make water and an ester.
Ethers:
consists
of two hydrocarbons joined by a oxygen between the them. Naming theses
two compounds is as easy as naming the two alkyl groups (in alphabetical
order) and ether at the end. An example is ethyl ether propyl, the
structural formula is CH3CH2OCH2CH2CH3. Another example is ethyl ethyl
ether more commonly named as diethyl ether. The common formula for
ethers is R-O-R, this means there are two alkyl groups (aka residue
groups).
Carboxylic Acids:
consists
of an alkyl group missing three hydrogen that were replaced by double
bonded oxygen and a hydroxide group. The common formula for carboxylic
acids is R-COOH. Naming carboxylic acids is not hard, starting with
propane and replaces the suffix –ane with -anoic acid, this compound is
propanoic acid and has the structural formula of CH3CH2COOH. Some
characteristics of this compound are reacting with alcohols to form
water and an ester. Carboxylic acids also have a similar property to
that of alcohols; both alcohols and carboxylic acids form weak acidic
solutions in water.
Ester:
are
generally formed through the reaction of carboxylic acids and alcohols.
Esters are the reason why certain foods have certain scents; this makes
the useful with perfumes. The general formula for an ester is R-COOR;
naming esters is a little harder than naming other compounds but
basically the alcohol gets the alkyl name ending and the carboxylic acid
gets the –anoate i.e. methanol reacting with butanoic acid to make
water and methyl butanoate; structural formula being CH3COOCH2CH2CH2CH3.
Aldehydes:
have
the general formula R-CHO. Naming these compounds should not be
confused with alcohols; aldehydes have the –al suffix while alcohols
have the –ol suffix. For example methanal (aka formaldehyde) is the
simplest aldehyde.
Ketones:
have
the general formula R-COR and contain the suffix –one. An example of a
ketone is butanone also know as methyl ethyl ketone to distinguish where
the oxygen is located. Other was of identifying the location of the
oxygen is by naming each carbon numerically and adding the number of the
carbon, where the oxygen is found, to the formula, such as 2-butanone.
Halocarbons:
are
essentially hydrocarbons where hydrogens have been replaced by
halogens. When the halogens are in the compound you change the name,
fluorine to fluoro, chlorine to chloro, bromine to bromo, and iodine to
iodo. An example of this compound is 2-choloropropane, CH3CHClCH3. Like
ketones, you should denote where the halogen is located in the compound
by numbering the carbons in the compound.
Amines:
contain
a nitrogenous base and have the structural formula R-NH2. These
compounds have the ability of making weak basic solutions by attracting
hydrogen to form A R-NH3+. Naming this compounds just requires the
addition of the suffix –amine after the alkyl name i.e. methylamine,
CH3NH2.
Cycloalkanes
Cycloalkanes
are alkanes that have ring, or cyclic structures. They have the
molecular formula CnH2n. Because the atoms are arranged in a ring shape,
the bond angles are less than they would be in ideal sp3 carbon. This
is referred to as ring strain. As a result of ring strain, the bond
energies of cycloalkanes are much less than those of regular alkanes. In
order to avoid ring strain, cycloalkanes in nature are often found in
different conformations. One example is the chair conformation of
cyclohexane.
Cyclohexane
Cyclohexane, C6H12, is a cycloalkane with 6 carbons.
Cyclohexane
is rarely found in its ring structure though because of its ring
strain. More often it is found in a chair conformation. The bond angles
in the chair conformer are very close to the ideal bond angles of sp3
carbons. About 110.9o compared to 109.5o (25) which is why this
conformation is more stable than the ring structure.
Benzene
Benzene
is a very important organic compound that is commonly used in
manufacturing and as an organic solvent in labs. Its molecular formula
is C6H6. In your organic chemistry textbook, you may see it presented in
a ring structure:
It may be
confusing as to how these "lines" can be a molecular structure. This
notation is a quick method for chemists to draw organic molecules
because the molecules can be very complex. In every corner, there is a
carbon, bonded to hydrogen because carbon likes to make 4 bonds.
This is what the above structures of benzene actually represent:
Isomers and Stereochemistry
The
structures of organic molecules are very important in considering their
properties. Most organic molecules display geometric properties such as
isomerism. Isomers are compounds with identical molecular formulas, but
with different structures. There are 2 kinds of isomers: constitutional
isomers and stereoisomers. Constitutional isomers differ in the
connectivity of their atoms. Stereoisomers are compounds that maintain
the same connectivity, but differ in the way their atoms are spatially
arranged. There are 2 kinds of stereoismers: cis-trans isomers and
enantiomers. Cis-trans isomers have different orientations of their
functional groups. In a cis isomer, the highest priority functional
groups are located on the same side of the double bond. In a trans
isomer, the highest priority functional groups are located on opposite
sides of the double bond. Enantiomers are compounds that have
nonsuperimposable mirror images of each. A molecule or object that has a
nonsuperimposable mirror image is chiral.
Constitutional Isomers: CH3CH2OH and CH3OCH3
Cis-trans Isomers:
Enantiomers:
Chirality and Enantiomers
In
organic chemistry, the concept of chirality is often applied in
medicine or compounds because the D-Dopa and the L-Dopa have different
functions, one of them does the desired function, while the other one
does not. This is because many biological receptor proteins in the body
are shaped so that only one enantiomer can bind to the substrate, while
the other cannot fit. An example is sugar in biological systems, sugar
is preferred in the D-Dopa form because in biological system, amino
acids is preferred in the L-Dopa. The mirror images of the molecules
does not have the same function as the other, they are usually called
chiral. The two chiral molecules are called enantiomers. Non-chiral
molecules are molecules that have a plane of symmetry in them and they
are called achiral.
References
McMurry, John. Organic Chemistry Fourth Edition. Pacific Grove, CA: Brooks/Cole Publishing Company, 1995.
Bruice, Paula Y. Essential Organic Chemistry. 2nd. Upper Saddle River, NJ: Prentice Hall, 2010.
ليست هناك تعليقات:
إرسال تعليق