Physical Properties of Aldehydes and Ketones

4 Key excerpts on "Physical Properties of Aldehydes and Ketones"

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  eBook - ePub

  Principles of Organic Chemistry

  • Robert J. Ouellette, J. David Rawn(Authors)
  • 2015(Publication Date)
  • Elsevier

    (Publisher)

  Table 10.1 ) because of dipole-dipole intermolecular forces due to the carbonyl group. However, alcohols have higher boiling points than aldehydes and ketones even though alcohols have smaller dipole moments than carbonyl compounds. This order of boiling points is the result of hydrogen bonding in alcohols that is not possible in carbonyl compounds. As the molecular weights of the carbonyl compounds increase, their dipole-dipole attractive forces become less important than the London forces of the hydrocarbon skeleton. As a result, the Physical Properties of Aldehydes and Ketones become more like those of hydrocarbons as chain length increases. The boiling point differences become smaller, although the order of boiling points is still alcohol > carbonyl compound > alkane.

  Solubility of Aldehydes and Ketones in Water

  Aldehydes and ketones cannot form hydrogen bonds with one another because they cannot function as hydrogen bond donors. However, the carbonyl oxygen atom has lone pair electrons that can serve as hydrogen bond acceptors. Thus, carbonyl groups can form hydrogen bonds with water. Hence, the lower molecular weight compounds formaldehyde, acetaldehyde, and acetone are soluble in water in all proportions.

  However, the solubility of carbonyl compounds in water decreases as the chain length increases, and their solubilities become more like those of hydrocarbons. Both acetone and 2-butanone (known in industry as methyl ethyl ketone, or MEK) are excellent solvents for many organic compounds. These polar solvents dissolve polar solutes because “like dissolves like.” These solvents also readily dissolve protic solutes such as alcohols and carboxylic acids because the carbonyl group acts as a hydrogen bond acceptor for these compounds, as shown in the molecular model of acetone hydrogen bonded to methanol, below.

  10.4 Oxidation-Reduction Reactions of Carbonyl Compounds

  The carbonyl group is in an oxidation state between that of an alcohol and a carboxylic acid. Thus, a carbonyl group can be reduced to an alcohol or oxidized to a carboxylic acid.

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  eBook - ePub

  The Chemistry of Carbonyl Compounds and Derivatives

  • Paulo Costa, Ronaldo Pilli, Sergio Pinheiro, Peter Bakuzis(Authors)
  • 2022(Publication Date)
  • Royal Society of Chemistry

    (Publisher)

  1 Aldehydes, Ketones, Imines and Nitriles

  This chapter introduces the reader to some of the concepts that will be used and discussed in more detail throughout the book. It begins with a brief mention of some of the compounds that contain aldehyde, ketone, imine, or nitrile groups that are produced at large or small scales by the chemical industry or are present in natural products with biological activity. This is followed by a description of the three-dimensional structures of the very simplest representatives of each functional group. Their configuration is then analyzed by both valence bond (VB) and simple molecular orbital (MO) approaches. This lays the framework for explaining some physical properties, such as dipole–dipole interactions, boiling and melting points, etc. More importantly, by stressing the relative size of the atomic coefficients of the HOMOs and LUMOs, the electrophilic nature of the carbon in each of the functional groups and the ability of the heteroatom to interact with Brønsted and Lewis acids is predicted. This is done from a historical perspective, which includes biographical sketches of some of those responsible for the development of our understanding of the structures of organic compounds.

  Key Concepts

  - Structural properties (bond lengths and bond angles) and three-dimensional structures for aldehydes, ketones, imines, and nitriles

  - Aldehydes, ketones, imines, and nitriles in natural products, drugs, and industrial chemicals

  - Correlation of dipole moment and physical properties (melting point, boiling point, solubility)

  - Structural description by valence bond (VB) and molecular orbital (MO) theories set the stage for discussions of their chemical reactivity

  1.1 Sources and Uses

  In this section, we will focus on the C=O, C=N, and C≡N functional groups present in the structure of natural products and some synthetic products of industrial relevance.

  Formaldehyde, the simplest carbonyl compound, is produced by methanol oxidation. In addition to its use in the preservation of biological species, a large amount of formalin (37% aqueous solution of formaldehyde) is produced for the plastic and resin industries, e.g. for the production of Bakelite® , a urea-formaldehyde resin, and other polymers.

  Acetaldehyde produced by the palladium-catalyzed ethylene oxidation (Wacker oxidation) is mainly used in the production of pharmaceutical compounds and polymers.

  Acetone (propanone) is produced by the oxidative cleavage of cumene (isopropyl benzene) and is, by far, the most important commercially available ketone. Together with butanone, they are industrial solvents with low toxicity and are easily distilled off due to their low boiling points. Acetone is also used as a solvent in enamels, paints, and varnishes and is an important raw material in the production of methacrylates and bisphenol.

  While formaldehyde and acetaldehyde are gases with irritating odors, some of their higher homologs have pleasant and varied smells: butyraldehyde (butter smell), heptanal (green-herbal odor), octanal (orange smell), nonanal (rose smell), citral and neral (citrus scent) are highly valued products in the perfume industry (Figure 1.1

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  eBook - ePub

  Handbook of Industrial Hydrocarbon Processes

  • James G. Speight(Author)
  • 2019(Publication Date)
  • Gulf Professional Publishing

    (Publisher)

  Aromatic hydrocarbon derivatives are derived from benzene. Group members have six free valence electrons which are distributed in a circle in the form of a charged cloud. Because of the presence of these valence electrons, we can predict that the reactivity of these aromatic compounds will be similar to other unsaturated hydrocarbon derivatives. However, benzene is much less reactive than other unsaturated hydrocarbon derivatives. Only at high temperatures and in the presence of a catalyst can benzene take on another hydrogen atom. When it does, cyclohexane is the resultant product.

  5. Physical properties

  Physical properties can be observed or measured without changing the composition of matter. Physical properties are used to observe and describe matter (Howard and Meylan, 1997 ; Yaws, 1999 ). The three states of matter are: solid, liquid, and gas. The melting point and boiling point are related to changes of the state of matter. All matter may exist in any of three physical states of matter. A physical change takes place without any changes in molecular composition. The same element or compound is present before and after the change. The same molecule is present throughout the changes. Physical changes are related to physical properties since some changes require a change in the three-dimensional structure of the molecule.

  Physical properties that are of interest in the current context include: (i) boiling point, (ii) density and specific gravity, (iii) dew point, (iv) flash point and ignition temperature, (v) melting point, and (vi) vapor density. These properties are listed in alphabetical order rather than attempt to assign importance to any individual property. The properties present indications the behavior of hydrocarbon derives as determined by application of standard test method (Speight, 2015 ).

  The physical properties of alkene derivatives are similar to those of the alkane derivatives. The boiling points of straight-chain alkenes increase with increasing molar mass, just as with alkanes. For molecules with the same number of carbon atoms and the same general shape, the boiling points usually differ only slightly, just as would be expected for chemicals in which whose molar mass differs by only one to two hydrogen atoms (i.e., RCH2 CH

  CH2 compared to RCH2 CH2 CH3

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  eBook - ePub

  Handbook of Industrial Hydrocarbon Processes

  • James G. Speight(Author)
  • 2010(Publication Date)
  • Gulf Professional Publishing

    (Publisher)

  cyclic alkanes) are differentiated from aliphatic hydrocarbons insofar as they contain a ring structure and form a homologous group of compounds. The first member of the series is cyclopentane followed by cyclohexane. Cycloalkanes are saturated compounds and, like linear alkanes, are not very reactive.

  Aromatic hydrocarbons are derived from benzene. Group members have six free valence electrons which are distributed in a circle in the form of a charged cloud. Because of the presence of these valence electrons, we can predict that the reactivity of these aromatic compounds will be similar to other unsaturated hydrocarbons. However, benzene is much less reactive than other unsaturated hydrocarbons. Only at high temperatures and in the presence of a catalyst can benzene take on another hydrogen atom. When it does, cyclohexane is the resultant product.

  5. Physical properties

  Physical properties can be observed or measured without changing the composition of matter. Physical properties are used to observe and describe matter (Howard and Meylan, 1997 and Yaws, 1999 ). The three states of matter are: solid, liquid, and gas. The melting point and boiling point are related to changes of the state of matter. All matter may exist in any of three physical states of matter.

  A physical change takes place without any changes in molecular composition. The same element or compound is present before and after the change. The same molecule is present throughout the changes. Physical changes are related to physical properties since some measurements require that changes be made.

  Physical properties that are of interest in the current context include: boiling point, melting point, density, vapor density, flash point, ignition temperature, and dew point.

  5.1. Boiling points and melting points

  The boiling point of an organic compound is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid.

  The melting point

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