Structure And Reactivity: Acids And Bases, Polar And Nonpolar Molecules

CHAPTER 2 Structure and Reactivity: Acids and Bases, Polar and Nonpolar Molecules

2-1

Kinetics and Thermodynamics of Simple Chemical Processes Chemical thermodynamics: Is concerned with the extent that a reaction goes to completion. Chemical kinetics: Is concerned with the speed that a reaction goes to completion. Thermodynamic Control: The most stable products are formed. Kinetic Control: The fastest product is formed.

Equilibria are governed by the thermodynamics of chemical change. When the concentrations of reactants and products no longer change, the system is said to be at equilibrium. A system at equilibrium is described mathematically: A 

B,

K =

[B ] [A ]

or A + B 

C + D,

K =

[C ][D ] [A ][B ]

A large value of K indicates that the reaction goes to completion.

The equilibrium constant can be related to the thermodynamic function Go:

 G   RT ln K o

When Go is negative, the equilibrium constant is greater than 1 and the products of the reaction are favored over the reactants. When Go is positive, the equilibrium constant is less that 1 and the reactants of the reactions are favored over the products.

The free energy change is related to changes in bond strengths and the degree of order in the system. The Gibbs free energy change for a reaction is related to the enthalpy change and the entropy change for the reaction:

G

 H  T S

o

o

o

The enthalpy change, Ho, can be estimated:

H

o



  B ond E nergies     B ond E nergies  R eactants

Products

The entropy change, So, is related to the amount of disorder in the system. The entropy of a substance phase is much larger in the gas than in the liquid phase.

In a chemical reaction where all substances are in the same phase, the entropy will increase if there are more product molecules than reactant molecules.

The rate of a chemical reaction depends on the activation energy. The potential energy of the system as a chemical change occurs can be plotted vs. time:

The higher the activation energy, Ea, the slower the reaction.

Collisions supply the energy to get past the activation-energy barrier. The average kinetic energy of molecules at room temperature is about 0.6 kcal mol-1. The kinetic energies of individual molecules can be plotted as a Boltzmann distribution curve:

As can be seen from the curves, there are more molecules having large kinetic energies at high temperature than at low temperature.

Since the energy required to reach the transition state in a chemical reaction comes from molecular collisions, the rate of chemical reactions always increases with rising temperatures.

The concentration of reactants can affect reaction rates. The rate of a chemical reaction can be expressed as a rate law. The rate law must be experimentally determined; it cannot be derived directly from the balanced chemical equation. F irst O rd er R eactio n : A  B , R ate = k [A ] S eco n d O rd er R eactio n : A + B  C + D , R ate = k [A ][B ]

The Arrhenius equation describes how temperature affects reaction rates. The rate constant, k, depends upon temperature according to the Arrhenius equation:

k  Ae

 E a / RT

In general, raising the reaction temperature by 10 oC will increase the rate constant by a factor of 2 or 3.

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Acids and Bases; Electrophiles and Nucleophiles

Acid and base strengths are measured by equilibrium constants. Brønsted-Lowry acid: a proton donor. Brønsted-Lowry base: a proton acceptor. In water:

Water itself is both an acid and a base: +

H 2O + H 2O 

H 3O + O H 

+



K W = [H 3 O ][O H ] = 1.0 x 10

 14

o

at 25 C

The pH of a solution is defined: pH =  log[H 3 O ] +

In pure water, the pH is 7. In acidic solutions the pH is less than 7 and in basic solutions the pH is above 7. The behavior of a general acid, HA, in water can be described: H A + H 2O  +

KA=

+

H 3O + O H 

[H 3 O ][A ] [H A ]



,

pK A =  log(K A )

The pKA is the pH at which the acid is 50% dissociated. If the pKA is less than 1, the acid is termed strong; if greater than 4, weak.

We can estimate relative acid and base strengths. The relative strength of a weak acid, HA, increases with: •Increasing electronegativity of A (CH4 < NH3 < H2O < HF) •Increasing size of A (HF < HCl < HBr < HI) •Resonance in A-

The same molecule can act as both an acid and as a base: HNO3 + H 2 0 



N O 3 + H 2O

H 2 SO 4 + H N O 3 



+

+

H SO 4 + H 2 N O 3

Lewis acids and bases interact by sharing an electron pair. Lewis Acid: A species containing an atom that is at least two electrons short of a closed outer shell.

Lewis Base: A species containing at least one lone pair of electrons.

A Lewis base shares its lone pair with a Lewis acid to form a new covalent bond:

  H  H  O   H H + O   +



The dissociation of a Brønsted acid is the reverse of the association of a Lewis acid and a Lewis base:

Note that the curved arrow points to the departing anion.

Electrophiles and nucleophiles also interact through movement of an electron pair. Many processes in organic chemistry exhibit characteristics of acid-base reactions. 2 C H 3C l + N aO H    C H 3 O H + N a C l

H O ,Δ

The carbon atom in chloromethane is termed electrophillic. The oxygen in the hydroxide is termed nucleophillic. The reaction of chloromethane and hydroxide ion is an example of a nucleophillic substitution reaction.

The terms Lewis acid and nucleophile are synonymous.

Nucleophilic substitution is a general reaction of the class of compounds called haloalkanes. Additional examples include:

The C-X bond in these examples constitutes the functional group, or center of chemical reactivity for the haloalkane class of organic compounds.

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Functional Groups: Centers of Reactivity

Functional groups are groups of atoms at sites of comparatively high chemical reactivity. They control the reactivity of the molecule as a whole.

Hydrocarbons are molecules that contain only hydrogen and carbon. Alkanes are compounds of hydrogen and carbon which contain only single bonds. When the carbon atoms form a ring, the compounds are called cycloalkanes.

Alkenes are hydrocarbons containing one or more C-C double bonds while alkynes contain one or more C-C triple bonds:

Benzene, C6H6, and its derivatives are examples of the class of organic compounds called aromatic:

Many functional groups contain polar bonds Haloalkanes have already been introduced. Alcohols (C-O-H) and ethers (C-O-C) can be converted into a large variety of other functionalities:

The carbonyl functionality, C=O, is found in aldehydes, ketones, and carboxylic acids:

Alkyl nitrogen and sulfur compounds are named amines and thiols:

R represents a part of an alkane molecule in the following common functional groups:

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Straight-Chain and Branched Alkanes

Alkanes can be classified into three general classes: straight-chain alkanes, branched alkanes, and cycloalkanes:

Straight-chain alkanes form a homologous series. The general formula of a straight chain alkane is CnH2n+2. Each member of the series differs from the previous one by the addition of a –CH2-, or methylene, group. Molecules related in this manner are called homologs of each other and the series is called a homologous series.

Branched alkanes are constitutional isomers of straight-chain alkanes. Branched chain alkanes have the same molecular formula as straight chain alkanes, CnH2n+2, but differ in connectivity. A branched and straight chain alkane are constitutional isomers of each other. There are three isomeric pentanes:

The number of isomeric alkanes increases dramatically with the number of carbon atoms:

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Naming the Alkanes

Many common or trivial names are still used widely used to name certain alkanes. Systematic IUPAC names are more precise. The first 20 straight chain alkanes are:

An alkyl group is formed by removing a hydrogen from an alkane. It is named by removing the –ane suffix and replacing it by –yl. CH3- methyl; CH3CH2- ethyl; CH3CH2CH2- propyl Additional prefixes are also used: sec- (or s-) for secondary, and tert- (or t-) for tertiary. A secondary carbon is directly attached to two other carbons. A tertiary carbon is directly attached to three other carbons.

Some common branched alkyl groups are:

For systematically naming branched alkanes, four IUPAC rules are used: IUPAC RULE 1: Find the longest chain (stem) in the molecule and name it. Groups other than hydrogen attached to this chain are called substituents. If the molecule has two or more stems of equal length, the one with the most substituents is the base stem chain.

IUPAC RULE 2: Name all groups attached to the longest chain as alkyl substituents. If a substituents chain is branched, find the longest chain in the substituents and then name all of its substituents.

IUPAC RULE 3: Number the carbons of the longest chain beginning with the end closest to a substituents.

If there are two substituents at equal distance from the ends of the chain, assign the lower number to the substituents coming first in alphabetical order.

If there are three or more substituents, the chain is numbered to give the lower number at the first difference between the two possible numbering schemes (first point of difference principle).

Substituents are numbered outward from the main chain. C1 will be the carbon attached to the main stem.

IUPAC RULE 4: Write the name of the alkane by first arranging all the substituents in alphabetical order. Precede each with the carbon number to which it is attached (to the stem) and a hyphen. When a molecule contains more than one instance of a particular substituents, precede the name by the attachment positions separated by commas and a prefix: di, tri, tetra, etc. In general, the Greek prefixes, sec- and tert- are not considered in the alphabetical ordering process.

If a particular complex substituents is present more than once, the prefixes bis, trix, tetrakis, pentakis, etc. are used. Remember the substituents carbon number 1 is the one attached to the stem chain.

Haloalkanes are named treating the halogen as a substituents to the longest stem, as for other substituents:

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Structural and Physical Properties of Alkanes

Alkanes exhibit regular molecular structures and properties. Alkane structures are regular. The carbon atoms are tetrahedral (bond angles close to 109o), C-C bond lengths all ~1.54 Å, and CH bond lengths all ~ 1.10 Å. The 3-D structures are depicted by the dashed/wedged link notation.

The physical constants of alkanes follow predictable trends:

Attractive forces between molecules govern the physical properties of alkanes. The physical forces between alkane molecules are due to London forces. These forces arise from the correlation of electron motion on neighboring molecules.

London forces are very weak and fall off with the 6th power of the distance between molecules (Coulomb forces fall off with the 2nd power).

With alkanes, London forces and therefore melting points increase with increasing molecular size (increased surface area contacts). Branched alkanes have smaller surface areas than their linear isomers and also cannot pack together as efficiently. Their melting points are usually lower than the corresponding linear isomers. Highly compact branched molecules (symmetrical) are exceptions. In addition there is a slight difference between odd and even numbered alkane chains. The odd numbered chains cannot pack as well in the solid, and therefore have slightly lower melting points than otherwise expected.

2-7

Rotation about Single Bonds: Conformations

Rotation interconverts the conformations of ethane. The barrier to rotation of the two methyl groups in ethane is approximately 2.9 kcal/mol. Since this amount of energy is readily available at room temperature from molecular collisions, the methyl groups are said to have free rotation. The rotation motions within ethane can be represented by the dashed/wedged notation:

During the rotation, the conformation moves from the staggered to the eclipsed, and to a second staggered conformation.

Newman projections depict the conformations of ethane. The Newman projection is an alternative to using the dashed/wedged notation:

The rotamers of ethane have different potential energies. The point of lowest potential during the C-C bond rotation in ethane is at the staggered conformation, the highest potential energy is at the eclipsed conformation (about 2.9 kcal/mole higher). The lifetime of the eclipsed conformation is extremely short and this conformation represents a transition state connecting two staggered conformations.

2-8

Rotation in Substituted Ethanes

Steric hindrance raises the energy barrier to rotation. The potential energy diagram for C-C bond rotation in propane shows steric hindrance. The energy of the eclipsed conformation is 3.2 kcal/mole above that of the staggered conformation.

There can be more than one staggered and one eclipsed conformation: conformational analysis of butane. There are two different types of staggered arrangements in butane. One in which the two terminal methyl groups are 180o apart (Anti), and two in which they are 30o apart (Gauche). These are connected by two types of eclipsed conformations, one in which the two methyl groups pass each other, and two in which the methyl groups pass by hydrogen atoms:

The transition state (eclipsed conformation) energies are 3.6 kcal/mol and 4.0 kcal/mol in butane:

The most stable anti conformation represents about 72% of the confomers present at 25% while the less stable gauche conformation represents about 28% of the confomers present.

2

Important Concepts 1. Chemical Reaction – An equilibrium controlled by thermodynamic and kinetic parameters. The Gibbs Free Energy: Go = -RT ln(K) = Ho - TSo. Ho: Related to bond energies of products and reactants. Exothermic: Ho < 0. (BE products < BE reactants) Endothermic: Ho > 0. (BE products > BE reactants)

So: Related to change in disorder.

2. Reaction Rate – Depends upon concentration of reactants, activation energy, and temperature. k = A exp(-Ea/RT)

3. Reaction Order – Cannot determine from balanced chemical equation. 1st: Order: rate depends upon one reactant concentration. 2nd Order: rate depends upon two reactant concentrations.

2

Important Concepts 4. BrØnsted and Lewis Acids and Bases BrØnsted acids and bases are proton donors and acceptors. Lewis acids and bases are electron pair acceptors and donors. Ka measures acid strength. pKa = - log(Ka).

5. Organic Molecule – A carbon skeleton with 6.

attached functional groups. Hydrocarbons – Composed of carbon and hydrogen only. Alkanes possess only single bonds and do not contain functional groups. Alkanes may be straight chain, branched, or cyclic. Straight chain and branched alkanes have the formula: CnH2n+2.

7. Homologs – Differ only in the number of methylene groups in the chain.

2

Important Concepts 8. Primary Carbon – Attached to only one other

9.

carbon. Secondary Carbon – Attached to two other carbons. Tertiary Carbon – Attached to three other carbons. IUPAC Rules – Naming saturated hydrocarbons: 1. 2. 3. 4.

Name the longest continuous chain in the molecule. Name all attached groups as substituents. Number the carbon atoms of the longest chain. Name the alkane, citing all substituents as prefixes arranged in alphabetical order and preceded by numbers designating their positions.

10. Alkane Attractions – Weak London forces. Polar molecules through stronger dipole-dipole interactions, and salts through very strong ionic interactions.

2

Important Concepts 8. Conformations – Rotation about C-C single bonds. • • •

Substituents on adjacent carbons can be staggered or eclipsed. The eclipsed conformer is the transition state between two staggered conformations. The activation energy for rotation is the energy difference between the staggered and eclipsed states. If both carbons bear alkyl or other groups, additional conformers may be possible. • •

•

Those in close proximity (60o) are gauche. Those directly opposite (180o) are anti.

Molecules adopt conformations of minimum steric hindrance.