chemistry

Entropy, free energy and thermodynamic equilibrium Entropy, Free Energy and Thermodynamic Equilibrium Chemical reactions are performed by mixing the reactants and regulating external conditions such as temperature and pressure . Two basic questions though arise: Is it possible for the reaction to occur at the selected conditions? If the reaction proceeds, what determines the ratio of products and reactants at equilibrium?   Both questions are answered by chemical thermodynamics : Thermodynamics can tell us whether a proposed reaction is spontaneous (possible) under particular conditions even before the actual experiment. Thermodynamics can also predict the ratio of products and reactants at equilibrium provided that the reaction is spontaneous . Note: Thermodynamics cannot answer though how fast a reaction will proceed.   After many years of observation scientists concluded that the characteristic common to all spontaneous...

Phase changes - energy changes - heating curves Phase Changes - Energy Changes - Heating Curves   Many important properties of liquids and solids relate to the ease with which they change from one state to another. Water for example, when heated it evaporates that is changes from liquid to the gas state. In general, each state of matter (solid, liquid, gas)  can change into either of the the other two states. Figure I.1 shows these transformations which are called phase changes or changes of state . What happens when a solid is heated? Typically, it melts to form a liquid. If the heating continues, the liquid at some point boils and forms the vapor phase (gas). This process can be represented by a heating curve : a plot of temperature versus time for a process where energy is added at a constant rate. The heating curve of water is shown in Fig. I.2. There are five separate zones on the graph (heating curve) of Fig. I.2:   Zone 1 (Ice) : As energy ...

The chemistry of enolate ions – Enolate ion reactions with alkyl halides The chemistry of enolate ions – Alkylations - Enolate ion reactions with alkyl halides Enolates are reactive nucleophiles . Although the major enolate Lewis contributor shows concentration of electron density on the electronegative oxygen (Fig. I.1) when it reacts as a nucleophile, it behaves like the electron density is concentrated on the α-carbon next to carbonyl group. However, from the resonance forms shown in Fig. I.1, it is clear that enolates are capable of reacting as both carbon and oxygen nucleophiles ( resonance structures II and I respectively). Enolates react with alkyl halides, aldehydes/ketones and esters and these reactions are shown below:   Reactions of enolates with alkyl halides Enolates are reactive nucleophiles and react with alkyl halides by the S N 2 and E2 mechanism . The electrophiles (alkyl halides in this case) need to be S N 2 reactive...

Food Chemistry: Antioxidants and Oxidation Reactions in Foods Home > Biological Chemistry > Food Chemistry > Food Chemistry: The browning reaction or reactions of amino acids during cooking > Caramelization in Cooking - Caramelization Reactions > Food Chemistry: Antioxidants and Oxidation Reactions in Foods Antioxidants and Oxidation Reactions in Foods An antioxidant is a substance that delays the onset or slows the rate of oxidation . It is also used to extent the self-life of a food . Antioxidants play an important role in food chemistry . Antioxidants can be divided into two categories: Naturally occurring ( vitamin E , vitamin C , β-carotene ) (Fig. I1) Synthetic ( 2-BHA , 3-BHA , BHT ) (Fig. I1) The unsaturated bonds present in all fats and oils represent active centers that react with oxygen. This reaction leads to the formation of primary, secondary, and tertiar...

Keto-Enol Tautomerism – Enolization – Reactions and Mechanism Acid or Base catalyzed Enolization – Reactions and Mechanism Carbonyl compounds (aldehydes, ketones, carboxylic esters, carboxylic amides) react as electrophiles at the sp 2 hybridized carbon atoms and as nucleophiles if they contain an H-atom in the α-position relative to their C=O or C=N bonds. This is because this H is acidic and it can be removed by a base leaving behind an electron pair for nucleophilic attacks .     For most compounds in organic chemistry all the molecules have the same structure – even if this structure cannot satisfactory represented by a Lewis formula – but for many compounds there is a mixture of two or more structurally distinct compounds that are in rapid equilibrium. This phenomenon is called tautomerism . Tautomerism is the phenomenon that occurs in any reaction that simply involves the intramolecular transfer of a proton. An equilibrium is established between the ...