Transcript 12.1 The Nature of Organic Molecules

12.1 The Nature of Organic Molecules
• Organic chemistry is the study of carbon
compounds.
• Carbon is tetravalent; it always forms four
bonds.
• Organic molecules have covalent bonds.
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12.1 The Nature of Organic Molecules
• When carbon bonds
to a more
electronegative
element, polar
covalent bonds
result.
• Organic molecules
have specific threedimensional shapes.
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Chloromethane, CH3Cl
12.1 The Nature of Organic Molecules
• Organic molecules often contain nitrogen and
oxygen in addition to carbon and hydrogen.
– Nitrogen can form single, double, and triple bonds to
carbon.
– Oxygen can form single and double bonds.
– Hydrogen can only form single bonds to carbon.
• Covalent bonding
– Individual molecules
– Lower melting and boiling points than inorganic salts
– Many organic compounds are liquids or low melting
solids at room temperature, and a few are gases.
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12.1 The Nature of Organic Molecules
• Most organic compounds are insoluble in water.
• Almost all of those that are soluble do not conduct
electricity.
• Only small polar organic molecules or large molecules
with many polar groups interact with water molecules
and thus, dissolve in water.
• Lack of water solubility for organic compounds has
important consequences.
• The interior of a living cell is a water solution that
contains many hundreds of different compounds. Cells
use membranes composed of water-insoluble organic
molecules to enclose their interiors and to regulate the
flow of substances across the cell boundary.
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12.2 Families of Organic Molecules: Functional Groups
• Organic compounds can be classified into
families according to structural features.
• The chemical behavior of family members is
often predictable based on their specific
grouping of atoms.
• There are a few general families of organic
compounds whose chemistry falls into simple
patterns.
• The structural features that allow classification of
organic compounds into families are called
functional groups.
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12.2 Families of Organic Molecules: Functional Groups
• A Functional group is an atom or group of
atoms within a molecule that has a characteristic
physical and chemical behavior.
• Each functional group is part of a larger
molecule, and a molecule may have more than
one class of functional group present.
• A given functional group tends to undergo the
same reactions in every molecule that contains
it.
• The chemistry of an organic molecule is
primarily determined by the functional groups it
contains, not by its size or complexity.
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12.2 Families of Organic Molecules: Functional Groups
Continued
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12.2 Families of Organic Molecules: Functional Groups
Continued
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12.2 Families of Organic Molecules: Functional Groups
• The first four families are hydrocarbons,
organic compounds that contain only carbon and
hydrogen.
– Alkanes have only single bonds and contain no
functional groups.
– Alkenes contain a carbon–carbon double-bond
functional group.
– Alkynes contain a carbon–carbon triple-bond
functional group.
– Aromatic compounds contain a six-membered ring of
carbon atoms with three alternating double bonds.
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12.2 Families of Organic Molecules: Functional Groups
• The next four families have functional groups that
contain only single bonds and a carbon atom bonded to
an electronegative atom.
– Alkyl halides have a carbon–halogen bond.
– Alcohols have a carbon–oxygen bond.
– Ethers have two carbons bonded to the same oxygen.
– Amines have a carbon–nitrogen bond.
• The next six families contain a carbon–oxygen double
bond: aldehydes, ketones, carboxylic acids, anhydrides,
esters, and amides.
• The remaining three families have functional groups that
contain sulfur: thioalcohols (known simply as thiols),
sulfides, and disulfides. These play an important role in
protein function.
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12.3 The Structure of Organic Molecules: Alkanes
and Their Isomers
• The general rule for all hydrocarbons except methane is
that each carbon must be bonded to at least one other
carbon.
• The carbon atoms bond together to form the “backbone”
of the compound, with the hydrogens on the periphery.
• The general formula for alkanes is CnHn+2 where n is the
number of carbons in the compound.
• As larger numbers of carbons and hydrogens combine,
the ability to form isomers arises.
• Compounds that have the same molecular formula but
different structural formulas are called isomers.
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12.3 The Structure of Organic Molecules: Alkanes
and Their Isomers
• There are two ways in which molecules that have the
formula C4H10 can be formed.
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12.3 The Structure of Organic Molecules: Alkanes
and Their Isomers
• A straight-chain alkane is an alkane that has
all its carbons connected in a row.
• A branched-chain alkane is an alkane that has
a branching connection of carbons.
• Constitutional isomers are compounds with
the same molecular formula, but with different
connections among their atoms.
• Constitutional isomers of a given molecular
formula are chemically distinct from one
another. They have different structures and
physical properties.
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12.3 The Structure of Organic Molecules:
Alkanes and Their Isomers
• When the molecular formula contains atoms
other than carbon and hydrogen, the
constitutional isomers obtained can also be
functional group isomers.
• These are isomers that differ in both molecular
connection and family classification.
• Ethyl alcohol and dimethyl ether both have the
formula C2H6O, but have very different
properties.
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12.4 Drawing Organic Structures
• A condensed structure is a shorthand
way of drawing structures in which C–C
and C–H bonds are understood rather
than shown.
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12.4 Drawing Organic Structures
• Occasionally, not all the CH2 groups
(called methylenes) are shown.
• CH2 is shown once in parentheses, with a
subscript indicating the number of
methylene units strung together.
CH3CH2CH2CH2CH2CH3 = CH3(CH2)4CH3
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12.4 Drawing Organic Structures
• A line structure (or line-angle structure)
is a shorthand way of drawing structures in
which carbon and hydrogen atoms are not
shown.
– A carbon is understood to be wherever a line begins or
ends and at every intersection of two lines.
– Hydrogens are understood to be wherever they are
needed to have each carbon form four bonds.
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12.4 Drawing Organic Structures
1. Each carbon–carbon bond is represented by a
line.
2. Anywhere a line ends or begins, as well as any
vertex where two lines meet, represents a
carbon atom.
3. Any atom, other than another carbon or a
hydrogen, attached to a carbon must be shown.
4. Since a neutral carbon atom forms four bonds,
all bonds not shown for any carbon are
understood to be the number of carbon–
hydrogen bonds needed to have the carbon
form four bonds.
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12.5 The Shapes of Organic Molecules
• Every carbon atom in an alkane has its four
bonds pointing toward the four corners of a
tetrahedron.
• The two parts of a molecule joined by a carbon–
carbon single bond in a noncyclic structure are
free to spin around the bond, giving rise to an
infinite number of possible conformations.
• The various conformations of a molecule are
called conformers.
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12.6 Naming Alkanes
• The system of naming (nomenclature) was
devised by the International Union of Pure and
Applied Chemistry, IUPAC.
• In the IUPAC system for organic compounds, a
chemical name has three parts: prefix, parent,
and suffix.
– The prefix specifies the location of functional groups
and other substituents.
– The parent tells how many carbon atoms are present
in the longest continuous chain.
– The suffix identifies to which family the molecule
belongs.
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12.6 Naming Alkanes
• Straight-chain alkanes are named by
counting the number of carbon atoms and
adding the family suffix -ane.
• Straight-chain alkanes have no
substituents, so prefixes are not needed.
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12.6 Naming Alkanes
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12.6 Naming Alkanes
• Substituents such as —CH3 and —
CH2CH3, that branch off the main chain
are called alkyl groups.
– Methyl group: —CH3
– Ethyl group: —CH2CH3
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12.6 Naming Alkanes
• The situation is more complex for larger
alkanes.
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12.6 Naming Alkanes
• There are four possible substitution
patterns for carbons attached to four
atoms.
– A primary (1°) carbon atom is a carbon
atom with 1 other carbon attached to it.
– A secondary (2°) carbon atom is a carbon
atom with 2 other carbons attached to it.
– A tertiary (3°) carbon atom is a carbon
atom with 3 other carbons attached to it.
– A quaternary (4°) carbon atom is a carbon
atom with 4 other carbons attached to it.
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12.6 Naming Alkanes
Branched-chain alkanes can be named by following four steps:
1. Name the main chain. Find the longest continuous chain of
carbons, and name the chain according to the number of
carbon atoms it contains. The longest chain may not be
immediately obvious.
2. Number the carbon atoms in the main chain, beginning at
the end nearer the first branch point.
3. Identify the branching substituents, and number each
according to its point of attachment to the main chain.
4. Write the name as a single word, using hyphens to separate
the numbers from the different prefixes and commas to
separate numbers if necessary. If two or more different
substituent groups are present, cite them in alphabetical
order. If two or more identical substituents are present, use
one of the prefixes di-, tri-, tetra-, and so forth, but do not use
these prefixes for alphabetizing purposes.
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12.7 Properties of Alkanes
• Alkanes contain only nonpolar C–C and C–H
bonds. The only intermolecular forces influencing
them are weak London dispersion forces.
• The effect of these forces is shown in the
regularity with which the melting and boiling points
of straight-chain alkanes increase with molecular
size.
– The first four alkanes, methane, ethane, propane, and
butane, are gases at room temperature and pressure.
– Alkanes with 5–15 carbon atoms are liquids.
– Alkanes with 16 or more carbon atoms are generally
low-melting, waxy solids.
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12.7 Properties of Alkanes
• Alkanes are insoluble in water but soluble in
nonpolar organic solvents.
• Because alkanes are generally less dense than
water, they float on its surface.
• Low-molecular-weight alkanes are volatile and must
be handled with care because their vapors are
flammable.
• Mixtures of alkane vapors and air can explode when
ignited by a single spark.
• Mineral oil, petroleum jelly, and paraffin wax are
mixtures of higher alkanes. All are harmless to body
tissue and are used in numerous food and medical
applications.
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12.7 Properties of Alkanes
Properties of Alkanes
• Odorless or mild odor, colorless, tasteless,
nontoxic
• Nonpolar, insoluble in water but soluble in
nonpolar organic solvents, less dense than
water
• Flammable, otherwise not very reactive
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12.8 Reactions of Alkanes
Combustion
• The reaction of an alkane with oxygen is called
combustion, an oxidation reaction that
commonly takes place in a controlled manner in
an engine or furnace.
• Carbon dioxide and water are the products of
complete combustion of any hydrocarbon, and a
large amount of heat is released.
• When hydrocarbon combustion is incomplete,
carbon monoxide and carbon-containing soot
are among the products.
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12.8 Reactions of Alkanes
Halogenation
• Halogenation is the replacement of an alkane hydrogen
by a chlorine or bromine initiated by heat or light.
• Halogenation is used to prepare a number of key
industrial solvents, as well as other molecules that are
used for the preparation of other larger organic
molecules.
• In a halogenation reaction, only one H at a time is
replaced. If allowed to react for a long enough time, all
H’s will be replaced with halogens.
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12.9 Cycloalkanes
• A cycloalkane is an alkane that contains
a ring of carbon atoms.
• To form a closed ring requires an
additional C–C bond and the loss of 2 H
atoms.
• The general formula for cycloalkanes is
CnH2n.
• Compounds of ring sizes from 3 through
30 and beyond have been prepared in the
laboratory.
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12.9 Cycloalkanes
• The C–C–C bond angles in cyclopropane
are 60°, and the bond angles in
cyclobutane are 90°, much less than the
ideal 109.5° tetrahedral angle.
• These compounds are less stable and
more reactive than other cycloalkanes.
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12.9 Cycloalkanes
Surprising Uses of Petroleum (Continued)
• Numerous types of products are made from
these petrochemicals:
– Lubricants, such as light machine oils, motor oils, and
greases
– Paraffin waxes
– Synthetic rubber
– Plastics
– Petroleum jelly, once considered a nuisance byproduct of oil drilling.
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12.10 Drawing and Naming Cycloalkanes
• Line structures are used almost
exclusively in drawing cycloalkanes, with
polygons used for the cyclic parts of the
molecules.
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12.10 Drawing and Naming Cycloalkanes
• Cycloalkanes are named by a straightforward
extension of the rules for naming open-chain
alkanes:
• STEP 1: Use the cycloalkane name as the parent. If
there is only one substituent on the ring, it is not
necessary to assign a number because all ring positions
are identical.
• STEP 2: Identify and number the substituents. Start
numbering at the group that has alphabetical priority,
and proceed around the ring in the direction that gives
the second substituent the lower possible number.
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Chapter Summary
1. What are the basic properties of organic
compounds?
•
Compounds made up primarily of carbon
atoms are classified as organic.
•
Many organic compounds contain carbon
atoms that are joined in long chains by a
combination of single double or triple bonds.
•
In this chapter we focused primarily on
alkanes, hydrocarbon compounds that contain
only single bonds between all C atoms.
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Chapter Summary, Continued
2. What are functional groups, and how are
they used to classify organic molecules?
•
Organic compounds can be classified into
various families according to the functional
groups they contain.
•
A functional group is part of a larger molecule
and is composed of a group of atoms that has
characteristic structure and chemical reactivity.
•
A given functional group undergoes nearly the
same chemical reactions in every molecule
where it occurs.
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Chapter Summary, Continued
3. What are isomers?
• Isomers are compounds that have the same
formula but different structures.
• Isomers that differ in their connections among
atoms are called constitutional isomers.
• When atoms other than carbon and hydrogen
are present the ability to have functional group
isomers arises.
• Isomers are molecules that, due to the
differences in their connections, have not only
different structures, but belong to different
families of organic molecules.
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Chapter Summary, Continued
4. How are organic molecules drawn?
• Organic compounds can be represented
by structural formulas in which all atoms
and bonds are shown.
• By condensed structures in which not all
bonds are drawn.
• By line structures in which the carbon
skeleton is represented by lines and the
locations of C and H atoms are
understood.
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Chapter Summary, Continued
5.
What are alkanes and cycloalkanes, and how are they
named?
•
Compounds that contain only carbon and hydrogen are called
hydrocarbons, and hydrocarbons that have only single bonds are called
alkanes.
A straight-chain alkane has all its carbons connected in a row, a
branched-chain alkane has a branching connection of atoms somewhere
along its chain, and a cycloalkane has a ring of carbon atoms.
Alkanes have the general formula CnH2n+2, whereas cycloalkanes have
the formula CnH2n.
Straight-chain alkanes are named by adding the family ending -ane to a
parent; this tells how many carbon atoms are present.
Branched-chain alkanes are named by using the longest continuous
chain of carbon atoms for the parent and then identifying the alkyl groups
present as branches off the main chain.
The position of the substituent groups on the main chain are identified by
numbering the carbons in the chain so that the substituents have the
lowest number.
Cycloalkanes are named by adding cyclo- to the name of the alkane.
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Chapter Summary, Continued
6. What are the general properties and
chemical reactions of alkanes?
• Alkanes are generally soluble only in
nonpolar organic solvents, have weak
intermolecular forces, and are nontoxic.
• Their principal chemical reactions are
combustion, a reaction with oxygen that
gives carbon dioxide and water, and
halogenation, a reaction in which
hydrogen atoms are replaced by chlorine
or bromine.
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