acid base nucleophiles.pdf

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Acidity and Basicity of Organic Molecules

Many important intermediates in chemical reactions are formed as a result ofmodifying a molecule or ion by the removal or addition of a proton.

The ease with which this can be done is described by the acidity or basicityconstant (Ka orKb) of the molecule.

It is possible to estimateKa orpKa values for molecules that are quite non-acidic

from a knowledge of the energy needed to remove a proton. Thus, whereas theKa of

acetic acid is 1.9E-5, theKa ofethane (CH3CH3) is about 1E-60. This means that it

is about 1055

times harder to remove a proton from ethane than it is to remove theproton from the carboxyl group of acetic acid.

Even though we can assign an acidity constant to ethane, we would never describe itas 'an acid'. A molecule is not described as being 'acidic' or 'basic' just becauseacidity or basicity constants are given for it!

Factors Affecting Acidity and Basicity

There are four important factors that help increase the ease with which a proton (H+)

can be removed from a molecule HA:

1. the electronegativity of the atom to which the H is attached2. the electronegativity of the atom(s) adjacent to the site of the H3. the hybridisation of the conjugate base

4. the extent to which the negative charge on the conjugate base (A-) can bedelocalised (resonance stability).

The Higher the Electronegativity, The more acidic the proton.

1.

Rank these molecules according to their relative pKa values on the scale shown

below.


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2.

Indicate which of the hydrogens would be easier to remove as a proton by clickingon the molecule showing the appropriate highlighted hydrogen.

CH3COOH CH3COOH

The H on the COOH is attached to a highly electronegative O and another O isattached to the adjacent carbon, therefor it is a clearly acidic H.The H on the CH3 atoms are attached to the C atoms which are only slightly moreelectronegative compared to H. Therefore they are slightly harder to remove, non-acidic.

The property of an acid is to :Donate a proton. "easier to remove"

Effect of Carbon Hybridisation 1

The effective electronegativity of a C atom in an organic molecule depends on itshybridisation state.

The greater the degree of p character in a hybrid orbital, thefurther away from

the C nucleus will be the 'centre of gravity' of the electron density.


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Conversely, the greater the s character of a hybrid orbital, the closer will be the'centre of gravity' of the electron density to the C nucleus.There fore a greater

electronegativity.

Charge Delocalisation Effects onAcidity

1.

2.


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Bond Making and Breaking

When a covalent bond in a molecule is broken, two outcomes are possible:

1) Homolytic Cleavage formation of radicals

2) Heterolytic Cleavage formation of ions

Determining which atom retains the two electrons from the covalent bond inheterolytic cleavage depens on the polarization of the bond.

Mechanistic Arrows

Chemists use a special arrow notation to show the movement of electrons during areaction. These arrows are called mechanistic arrows.

A single-sided curved arrow represents the movement of only one electronwhile

A double-sided curved arrow represents the movement of two electrons.

The correct way to draw mechanistic arrows is to show the arrow beginning from a

region of high electron density (such as a bond or a lone pair of electrons) and


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terminating at the new resting-place of the electrons, therebyforming a new bondor breaking an existing bond.

Types of Reagent

Most chemical reactions involve the interaction ofnucleophiles and electrophiles.

* Nucleophiles - (nucleus-loving) - ions or molecules that are able to donate a pairof electrons. Often these species have a lone pair of electrons, however, sometimesthey do not (e.g. the double bonds in alkenes are electron rich and often act asnucleophiles).

* Electrophiles - (electron-loving) - ions or molecules that can accept a pair ofelectrons. Often these species are positively charged (or+) or are electrondeficient (e.g. AlCl3).

Mechanistic arrows often begin at a nucleophile, orterminate at an electrophile.


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acid base nucleophiles.pdf

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