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POSDRU/156/1.2/G/135764

Curs VI

Influenta grupelor functionale asupra

biocompatibilitatii. Notiuni introductive (V)

The a-Hydrogen Is Acidic

the anion is stabilized

by resonance

A carbon acid is a compound with a relatively acidic

hydrogen bonded to an sp3-hybridized carbon

Tautomeria Enol-Cetona

An Enol Is a Better Nucleophile

Than an Alkene

Carbonyl compounds that form enol undergo substitution

reactions at the a-carbon: an a-substitution reaction

An Acid-Catalyzed

a-Substitution Reaction

A Base-Catalyzed

a-Substitution Reaction

An Enolate Is an Ambident Nucleophile

Reaction at the C or O site depends on the electrophile

and on the reaction condition

Protonation occurs preferentially on the O site

Otherwise, the C site is likely the nucleophile

Conversion of a Methyl Ketone to a

Carboxylic Acid

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Addition and elimination reactions are exactly opposite. A bond is formed in elimination reactions, whereas a bond is broken in addition reactions.

Reactia de eliminare

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Reactia de eliminare

In an elimination reaction, two groups X and Y are removed from a starting material.

Two bonds are broken, and a bond is formed between adjacent atoms.

The most common examples of elimination occur when X = H and Y is a heteroatom more electronegative than carbon.

Reactia de eliminare

Exercitii propuse:

Determinai pentru urmtoarele structuri, caracterul nucleofil sau electrofil, pentru atomii marcai:

Exercitii propuse:

Reactiile de oxidare si reducere

Commonly Termed REDOX Reactions

From General Chemistry, we Will Recall Oxidation: Loss of Electrons Reduction: Gain of Electrons

Organic Chemists will Typically use Different Definitions Reduction: Increase Hydrogen Content (Decrease Oxygen) Oxidation: Decrease Hydrogen Content (Increase Oxygen)

Oxidizing/Reducing Agents: Usually Inorganic Compounds (M+)

Some common reducing agents

Some common oxidizing agents

Oxidation results in an increase in the number of bondsbetween C and a more electronegative atom, CZ, (usually

CO) or decrease in the number of CH bonds.

Reduction results in a decrease in the number of CZ bonds(usually CO) or an increase in the number of

CH bonds.

Figure 12.1 A general scheme for the oxidation and reduction of a carbon compound

Reactiile de oxidare si reducere

Two carbon atoms may be involved in a single oxidation or reduction reaction.

The conversion of an alkyne to an alkene, or an alkene to an alkane are examples of this type reduction because each

process adds two new CH bonds to the starting material.

Figure 12.2 Oxidation and reduction of

hydrocarbons

Introduction:

There are three ways of introducing 2 Hs in a reduction.

1. The first method uses H2 gas. Catalytic hydrogenation requires a metal

catalyst (usually Pt, Pd or Ni) with H2.

2. A second method adds two protons and two electrons to a substrate, and 2H+

+ 2e- = H2Reductions of this sort use alkali metals as a source of electrons, and liquid

ammonia as a source of protons, Nao in NH3 (liq) or Lio in NH3 (liq).

These are called dissolving metal reductions.

Reducing Agents:

3. The third way is to add hydride (H) and a proton (H+).

The most common hydride reducing agents contain a hydrogen atom bonded to boron or aluminum. Simple examples include sodium

borohydride (NaBH4) and lithium aluminum hydride (LiAlH4).

NaBH4 and LiAlH4 deliver H to the substrate, and then a proton is added from H2O or an alcohol.

Reducing Agents:

The addition of H2 occurs only in the presence of a metal catalyst, and thus it is called catalytic hydrogenation.

The catalyst consists of a metal, usually Pd, Pt, or Ni, adsorbed onto a finely divided inert solid, such as charcoal.

H2 adds in a syn fashion.

Reduction of Alkenes: Catalytic Hydrogenation

Oxidation and Reduction

Reduction of Alkenes: Catalytic Hydrogenation

Alkyne reduction to a Trans Alkene:

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Alkyl halides can be reduced to alkanes with LiAlH4.

Epoxide rings can be opened with LiAlH4 to form alcohols.

Reduction of Polar CX Bonds:

Figure 12.6 Examples of reduction of C X bonds with LiAIH4

This reaction follows an SN2 mechanism.

Unhindered CH3X and 1 alkyl halides are more easily reduced than more substituted 2 and 3 halides.

In unsymmetrical epoxides, nucleophilic attack of H (from LiAlH4) occurs at the less substituted carbon atom.

Reduction of Polar CX Bonds:

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There are two main categories of oxidizing agents:

1. Reagents that contain an oxygen-oxygen bond

2. Reagents that contain metal-oxygen bonds

Oxidizing agents containing an OO bond include O2, O3(ozone), H2O2 (hydrogen peroxide), (CH3)COOH (tert-butyl

hydroperoxide), and peroxyacids.

Peroxyacids (or peracids) have the general formula RCO3H.

Oxidizing Agents:

Figure 12.7 Common

peroxyacids

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The most common oxidizing agents with metal-oxygen bonds contain either chromium +6 (six CrO bonds) or manganese +7 (seven MnO bonds).

Common Cr6+ reagents include CrO3 and sodium or potassium dichromate (Na2Cr2O7 and K2Cr2O7). Pyridinium chlorochromate

(PCC) is a more selective Cr6+ oxidant.

The most common Mn7+ reagent is KMnO4 (potassium permanganate).

Other oxidizing agents that contain metals include OsO4 (osmium tetroxide) and Ag2O [silver(I) oxide].

Oxidizing Agents:

Figure 12.8 Oxidation reactions of alkenes, alkynes, and alcohols

Oxidizing Agents: