[PDF] Chapter 24 The Chemistry of Life: Organic and Biological Chemistry

43845_7IRM_ch24_1.pdf The Chemistry of Life: Organic and Biological Chemistry

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Chapter 24. The Chemistry of Life: Organic and Biological

Chemistry

Common Student Misconceptions

• Students will often find the distinction between organic and inorganic molecules as somewhat vague.

As a rule of thumb, inorganic carbon is carbon that is not bound to hydrogen, for example, H 2 CO 3 .

• Students inter pret straight chain to mean geometrically linear. They need to be reminded of the

tetrahedral C atom from VSEPR theory.

• Students often thin k that chirality is only possible for organic compounds, and that chiral centers are

limited to C atoms. • Many st udents think that one cannot have aqueous reactions involving organic compounds.

• Students are surprised that, in organic chemistry, acid-base reactions are more often viewed as an

electron pair transfer from a Lewis base to a Lewis acid instead of a proton transfer from a B-L acid

to a B-L base.

• Students are surprised that the -OH (hydroxyl) group in organic chemistry is not the same as the OH

 ion in ionic compounds, such as metal hydroxides.

Teaching Tips

• Students should be encouraged to follow the chapter links to review earlier material as they progress

through this chapter. • In particular, students should review Lewis structures as well as Lewis acids and bases. • Students shou ld be encouraged to draw and re-draw structures of organic compounds.

Lecture Outline

24.1 General Characteristics of Organic Molecules

• Organic chemistry is the branch of chemistry that studies carbon compounds.

• Biochemistry, biological chemistry, or chemical biology is the study of the chemistry of living

things.

The Structures of Organic Molecules

1,2,3

• The shapes of organic and biochemical molecules are important in determining their physical and

chemical properties. • Consider the element carbon: • Using the VSEPR model we find that the bonds to carbon involve four electron pairs. • The electron pairs are in a tetrahedral arrangement when all four bonds are single bonds. • The carbon is sp 3 hybridized. • A carbon with one double bond shows a trigonal arrangement. • The carbon is sp 2 hybridized. • If the carbon has a triple bond, the arrangement is linear. • The carbon is sp hybridized. • C-H bonds occur in alm ost every organic molecule. 1 "Methane" 3-D Model from Instructor's Resource CD/DVD 2 "Acetonitrile" 3-D Model from Instructor's Resource CD/DVD 3 Figure 24.1 from Transparency Pack

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• Carbon-carbon bonds for t he backbone or skeleton of the molecule and the H atoms are on the surface

of the molecule.

The Stabilities of Organic Substances

• The stability of organic substances varies.

• Substances such as benzene have a special stability due to the delocalization of S electrons.

• A group of atoms that determines how an organic molecule functions or reacts is a functional group.

• Functional groups are the center of reactivity in organic molecules. • This group of atoms determines how an organic molecule reacts or functions. Solubility and Acid-Base Properties of Organic Substances 4,5 • The m ost common bonds in organic substances are carbon-carbon bonds. • This results in a low overall polarity of many organic molecules. • Such molecules are soluble in nonpolar solvents. • Organic substances that are soluble in water and other polar solvents have polar groups. • Examples: glucose and ascorbic acid (vitamin C).

• Soaps and detergents are examples of molecules that have both a polar part (which is water soluble)

and a nonpolar part (which is soluble in nonpolar substances such as fat). • They function as surfactants. • Many organic molecules contain acidic or basic groups. • Carboxylic acids contain the functional group -COOH. • Amines are important organic bases. • They contain the functional groups -NH 2 , -NHR, or -NR 2 . • "R" groups are groups consisting of carbon-carbon and carbon-hydrogen bonds. • Some m olecules contain both an acidic and a basic group.

24.2 Introduction to Hydrocarbons

6,7,8,9,10,11,12,13

• The si mplest class of organic molecules is the hydrocarbons. • Hydrocarbons consist only of carbon and hydrogen. • There are four major classes of hydrocarbons: alkanes, alkenes, alkynes, and aromatics. • Alkanes contain only single bonds.

• These compounds are also called saturated hydrocarbons because they have the largest possible

number of hydrogen atoms per carbon. • An example is ethane (C 2 H 6 ). • Alkenes contain at least one carbon-carbon double bond. • They are also called olefins. • An example is ethylene (C 2 H 4 ). • Alkynes contain a carbon-carbon triple bond. • An example is acetylene (C 2 H 2 ). 4 "Glucose" 3-D Model from Instructor's Resource CD/DVD 5 "Sodium Stearate" 3-D Model from Instructor's Resource CD/DVD 6 Table 24.1 from Transparency Pack 7 "Ethane" 3-D Model from Instructor's Resource CD/DVD 8 "Propane" 3-D Model from Instructor's Resource CD/DVD 9 "Ethene (ethylene)" 3-D Model from Instructor's Resource CD/DVD 10 "Ethyne (acetylene)" 3-D Model from Instructor's Resource CD/DVD 11 "Benzene" 3-D Model from Instructor's Resource CD/DVD 12 "Boiling Point" Activity from Instructor's Resource CD/DVD 13 "Alkanes: Abundant, Pervasive, Important, and Essential" from Further Readings The Chemistry of Life: Organic and Biological Chemistry

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• Aromatic hydrocarbons have carbon atoms connected in a planar ring structure. • The carbons are linked by both V and S bonds. • The best known example is benzene (C 6 H 6 ). • Alkenes, alkynes, and aromatic hydrocarbons are all examples of unsaturated hydrocarbons. • The name of the alkane varies according to the number of C atoms present in the chain. • We can make a table of members of a homologous series of straight-chain alkanes. • In this table, each member differs by one CH 2 unit. • The names each end in -ane. • The prefix assigned indicates the number of carbon atoms. • Example: CH 4 is the alkane with a single carbon atom; it is called methane. • The next member of the series is C 2 H 6 , with two carbon atoms; it is called ethane. • The form ulas for alkanes may be written in a notation called condensed structural formulas.

• This notation shows which atoms are bonded to one another, but does not require that we draw in

all of the bonds. • Notice that each carbon in an alkane has four single bonds.

Structures of Alkanes

14 • VSEPR theory predicts each C atom is tetrahedral. • Therefore, each C atom has sp 3 -hybridized orbitals. • Rotation about the C-C bond in alkanes is relatively easy.

Structural Isomers

15,16,17,18,19

• In straight-chain hydrocarbons, the C atoms are joined in a continuous chain.

• In a straight-chain hydrocarbon, no one C atom may be attached to more than two other C atoms.

• Straight chain hydrocarbons are not linear. • Each C atom is tetrahedral, so the chains are bent. • Branched-chain hydrocarbons are possible for alkanes with four or more C atoms. • Structures with different branches can be written for the same formula. • Structural isomers are compounds with the same molecular formula but different bonding arrangements. • Structural isomers have somewhat different physical and chemical properties.

Nomenclature of Alkanes

20,21,22,23,24,25

• Organic co mpounds are named according to rules established by the International Union for Pure and

Applied Chemistry (IUPAC).

• To name alkanes: • Find the longest chain and use it as the base name of the compound. 14 Figure 24.3 from Transparency Pack 15 Figure 24.4 from Transparency Pack 16 "Butane" 3-D Model from Instructor's Resource CD/DVD 17 "Methylpropane (isobutane)" 3-D Model from Instructor's Resource CD/DVD 18 "Pentane" 3-D Model from Instructor's Resource CD/DVD 19 "Neopentane (dimethylpropane)" 3-D Model from Instructor's Resource CD/DVD 20 "Nomenclature of Alkanes" Activity from Instructor's Resource CD/DVD 21
Table 24.4 from Transparency Pack 22
"The IUPAC Rules for Naming Organic Molecules" from Further Readings 23
"Why is 'R' Used to Symbolize Hydrocarbon Substituents?" from Further Readings 24
"A Simple Method of Drawing Stereoisomers from Complicated Symmetrical Structures" from Further

Readings

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"Condensed Structural Formula" Activity from Instructor's Resource CD/DVD

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• Groups attached to the main chain are called substituents. • Number the carbon atoms in the longest chain starting with the end closest to a substituent. • The preferred numbering will give substituents the lowest numbers. • Name and give the location of each substituent. • A substituent group formed by removing an H atom from an alkane is called an alkyl group. • Alkyl groups are named by replacing the -ane ending with -yl. • Example: CH 4 is methane and a -CH 3 group is a methyl group. • When two or more substituents are present, list them in alphabetical order.

• When there are two or more of the same substituent, the number of that type of substituent is

indicated by a prefix: (i.e., "dimethyl" indicates two methyl group substituents).

Cycloalkanes

• Alkanes that form rings are called cycloalkanes.

• Cyclopropane and cyclobutane are strained because the C-C-C bond angles in the ring are less than

the 109.5° required for a tetrahedral geometry. • Because of the strain in the ring, cyclopropane is very reactive.

Reactions with Alkanes

26
• The C-C and C-H bonds are very strong. • Therefore, alkanes are very unreactive. • At room temperature alkanes, do not react with acids, bases, or strong oxidizing agents. • Alkanes do undergo combustion in air (making them good fuels): 2C 2 H 6 (g) + 7O 2 (g)