Download Introduction to Organic Molecules

ORGANIC CHEMISTRY
Dr. Serkan SAYINER
[email protected]
Introduction to Organic Molecules
Characteristic Features of Organic Compounds, Drawing Organic Molecules, Functional Groups
Dr. Serkan SAYINER
[email protected]
Introduction to Organic Chemistry
 What is organic chemistry?
• Organic chemistry is the study of
compounds that contain the element
carbon.
 Clothes, foods, medicines, gasoline,
refrigerants, and soaps are composed
almost solely of organic compounds.
Introduction to Organic Chemistry
 Organic compounds exist as discrete molecules with much
weaker intermolecular forces—the forces that exist between
molecules—than those seen in ionic compounds, which are
held together by very strong interactions of oppositely
charged ions.
• As a result, organic compounds resemble other covalent compounds
in that they have much lower melting points and boiling points than
ionic compounds. While ionic compounds are generally solids at
room temperature, many organic compounds are liquids and some
are even gases.
Characteristic Features of Organic Compounds
 All organic compounds contain carbon atoms and most
contain hydrogen atoms.
 Carbon always forms four covalent bonds, and
hydrogen forms one covalent bond.
 Carbon is located in group 4A of the periodic table, so a
carbon atom has four valence electrons available for
bonding.
Characteristic Features of Organic Compounds
 Since hydrogen has a single valence electron, methane
(CH4) consists of four single bonds, each formed from
one electron from a hydrogen atom and one electron
from carbon.
Characteristic Features of Organic Compounds
 Carbon forms single, double, and triple bonds to other
carbon atoms.
 When a compound contains two or more carbon atoms,
the type of bonding is determined by the number of
atoms around carbon.
Characteristic Features of Organic Compounds
 A C atom surrounded by four atoms
forms four single bonds.
 In ethane (C2H6), each carbon atom is
bonded to three hydrogen atoms and
one carbon atom.
 All bonds are single bonds.
Characteristic Features of Organic Compounds
 A C atom surrounded by three
atoms forms one double bond.
 In ethylene (C2H4), each carbon
atom is surrounded by three atoms
(two hydrogens and one carbon);
thus, each C forms a single bond to
each hydrogen atom and a double
bond to carbon.
Characteristic Features of Organic Compounds
 A C atom surrounded by two atoms
generally forms one triple bond.
 In acetylene (C2H2), each carbon
atom is surrounded by two atoms
(one hydrogen and one carbon);
thus, each C forms a single bond to
hydrogen and a triple bond to
carbon.
Characteristic Features of Organic Compounds
 Some compounds have chains of atoms and some
compounds have rings.
 For example, three carbon atoms can bond in a row to
form propane, or form a ring called cyclopropane.
• Propane is the fuel burned in gas grills.
• Cyclopropane is an anesthetic.
Characteristic Features of Organic Compounds
 Organic compounds may also contain elements other
than carbon and hydrogen.
• Any atom that is not carbon or hydrogen is called a
heteroatom.
• Each heteroatom forms a characteristic number of bonds,
determined by its location in the periodic table.
• The common heteroatoms also have nonbonding, lone pairs of
electrons, so that each atom is surrounded by eight electrons.
Characteristic Features of Organic Compounds
 Nitrogen forms three bonds and has one lone pair of
electrons, while oxygen forms two bonds and has two
additional lone pairs. The halogens form one bond and
have three additional lone pairs.
 Except for hydrogen, these common elements in organic
compounds follow one rule in bonding:
Characteristic Features of Organic Compounds
 Oxygen and nitrogen form both single and multiple bonds
to carbon.
 The most common multiple bond between carbon and
a heteroatom is a carbon–oxygen double bond (C=O).
 The bonding patterns remain the same even when an
atom is part of a multiple bond.
• For example: Methanol (CH3OH) and Formaldehyde (H2C=O, a
preservative).
Drawing Organic Molecules
 Organic molecules often contain many atoms, so we
need shorthand methods to simplify their structures.
 The two main types of shorthand representations used
for organic compounds are;
• Condensed structures and
• Skeletal structures.
Condensed Structures
 Condensed structures are most often used for a
compound having a chain of atoms bonded together,
rather than a ring.
 The following conventions are used.
• All of the atoms are drawn in, but the two-electron bond
lines are generally omitted.
• Lone pairs on heteroatoms are omitted.
Condensed Structures
 To interpret a condensed formula, it is usually best to
start at the left side of the molecule and remember
that the carbon atoms must have four bonds.
• A carbon bonded to 3 H’s becomes CH3.
• A carbon bonded to 2 H’s becomes CH2.
• A carbon bonded to 1 H becomes CH.
Sometimes these structures are further simplified by using
parentheses around like groups. Two CH2 groups bonded
together become (CH2)2. Two CH3 groups bonded to the
same carbon become (CH3)2C.
Sample Problem
Convert each compound into a condensed structure.
Solution
Skeletal Structures
 Skeletal structures are used for organic compounds
containing both rings and chains of atoms.
 Three important rules are used in drawing them.
1. Assume there is a carbon atom at the junction of any two
lines or at the end of any line.
2. Assume there are enough hydrogens around each carbon to
give it four bonds.
3. Draw in all heteroatoms and the hydrogens directly bonded
to them.
Skeletal Structures
 Rings are drawn as polygons with a carbon atom
"understood" at each vertex, as shown for cyclohexane
and cyclopentanol.
• All carbons and hydrogens in these molecules are understood,
except for H’s bonded to heteroatoms.
Functional Groups
 In addition to strong C-C and C-H bonds, organic
molecules may have other structural features as well.
 Although over 20 million organic compounds are
currently known, only a limited number of common
structural features, called functional groups, are found
in these molecules.
Functional Groups
 A functional group is an atom or a group of atoms with
characteristic chemical and physical properties.
 A functional group contains a heteroatom, a multiple
bond, or sometimes both a heteroatom and a multiple
bond.
 A functional group determines a molecule’s shape,
properties, and the type of reactions it undergoes.
Functional Groups
 A functional group behaves the same whether it is
bonded to a carbon backbone having as few as two or
as many as 20 carbons.
• For this reason, we often abbreviate the carbon and hydrogen
portion of the molecule by a capital letter R, and draw the R
bonded to a particular functional group.
Functional Groups
 For example;
• Ethanol (CH3CH2OH), has two carbons and five hydrogens in
its carbon backbone, as well as an OH group, a functional
group called a hydroxyl group.
• The hydroxyl group determines the physical properties of
ethanol as well as the type of reactions it undergoes.
• Moreover, any organic molecule containing a hydroxyl group has
properties similar to ethanol. Compounds that contain a
hydroxyl group are called alcohols.
Functional Groups
 The most common functional groups can be subdivided
into three types.
1. Hydrocarbons
2. Compounds containing a single bond to a heteroatom
3. Compounds containing a C=O group
Hydrocarbons
 Hydrocarbons are compounds that contain only the
elements of carbon and hydrogen.
• Alkanes have only C-C single bonds and no functional group.
• Ethane, CH3CH3, is a simple alkane.
• Alkenes have a C=C double bond as their functional group.
• Ethylene, CH2=CH2, is a simple alkene.
• Alkynes have a C≡C triple bond as their functional group.
• Acetylene, HC≡CH, is a simple alkyne.
• Aromatic hydrocarbons contain a benzene ring, a six-
membered ring with three double bonds.
Ethane
Ethylene
Acetylene
Benzene
Hydrocarbons
 All hydrocarbons other than alkanes contain multiple bonds.
• Alkanes, which have no functional groups and therefore no reactive
sites, are notoriously unreactive except under very drastic
conditions.
• For example, polyethylene is a synthetic plastic and high molecular
weight alkane, consisting of long chains of —CH2— groups bonded
together, hundreds or even thousands of atoms long. Because it has
no reactive sites, it is a very stable compound that does not readily
degrade and thus persists for years in landfills.
Compounds Containing a Single Bond to a
Heteroatom
 Several types of functional groups contain a carbon
atom singly bonded to a heteroatom.
• Common examples include alkyl halides, alcohols, ethers, and
amines.
 Molecules containing these functional groups may be
simple or very complex.
• It doesn’t matter what else is present in other parts of the
molecule.
Compounds Containing a Single Bond to a
Heteroatom
 Always dissect it into small pieces to identify the
functional groups. For example;
• Diethyl ether, the first general anesthetic, is an ether
because it has an O atom bonded to two C’s.
• Tetrahydrocannabinol (THC), the active component in
marijuana, is also an ether because it contains an O atom
bonded to two carbon atoms. In this case the O atom is
also part of a ring.
Bromomethane
Methanol
Dimethyl ether
Methylamine
Methanethiol
Dimethyl sulfide
Compounds Containing a C=O Group
 Many different kinds of compounds contain a carbon–
oxygen double bond (C=O, carbonyl group).
 Carbonyl compounds include aldehydes, ketones,
carboxylic acids, esters, and amides.
 The type of atom bonded to the carbonyl carbon—
hydrogen, carbon, or a heteroatom—determines the
specific class of carbonyl compound.
Compounds Containing a C=O Group
 Take special note of the condensed structures used to
draw aldehydes, carboxylic acids, and esters.
 An aldehyde has a hydrogen atom bonded directly to
the carbonyl carbon.
Compounds Containing a C=O Group
 A carboxylic acid contains an OH group bonded directly
to the carbonyl carbon.
 An ester contains an OR group bonded directly to the
carbonyl carbon.
Acetaldehyde
Acetone
Acetic acid
Methyl acetate
Acetamide
Sample Problem
Identify the functional group in each compound.
It is a hydrocarbon
with a carbon–carbon
double bond, making
it an alkene.
It has a carbon atom
bonded to a hydroxyl
group (OH), making it
an alcohol.
It contains a C=O.
Since the carbonyl
carbon is bonded to
two other carbons in
the ring, it is a
ketone.
Reference Books
 Smith JG (2010). Organic Chemistry, 3rd Edition,
McGraw-Hill.
 Smith JG (2012). General, Organic, & Biological
Chemistry 2nd Edition, McGraw-Hill.