CSU Organic Chemistry - Functional Groups PDF

Properties of Hydrocarbons and Functional Groups By the end of this module, the student should be able to: 1. Classify organic molecules according to their functional groups. 2. Relate the structure and physical properties of organic molecules. 3. Predict the physical properties of organic chemi...

Properties of Hydrocarbons and Functional Groups By the end of this module, the student should be able to: 1. Classify organic molecules according to their functional groups. 2. Relate the structure and physical properties of organic molecules. 3. Predict the physical properties of organic chemicals based on their structures (e.g. relative boiling point, melting point, and solubility.) Review on Polar bonding and Intermolecular Forces Polar covalent bonds Ionic or covalent bonds In NaCl, sodium transfers an electron to chlorine to give Na ion and Cl ion, which are held together by electrostatic attractions between unlike charges. The C-C bond in ethane, however, is covalent. The two bonding electron are shared equally by the two equivalent carbon atoms, resulting in a symmetrical electron distribution.  As a rough guide, bonds between atoms whose electronegativities differ by less than 0.5 are nonpolar covalent 0.5-2 are polar convalent More than 2 are largely ionic C-H bonds are nonpolar Oxygen carbon and nitrogen carbon are polar Dipole moments Just as individual bonds are often polar, molecules as a whole are often polar also. Molecular polarity results from the vector summation of all individual bond polarities and lone-pair contributions in the molecule. As a practical matter, strongly polar substance are often soluble in polar solvents like water, whereas less polar substances are insoluble in water  Net molecular polarity is measure by a quantity called dipole moment. Non-covalent Interactions between molecules Dipole-dipole forces Occurs between polar molecules because of electrostatic interaction among dipoles. Dispersion forces Occur between all neighboring molecules and arise because the electron distribution within molecules is constantly changing.  Hydrogen bond An attractive interaction between a hydrogen bonded to an electronegative O or N atom and an unshared electron pair on another O or N atom. Causes water to stay liquid Hold enzymes in shape Cause strand of DNA to pair up and coil into the double helix Hydrophilic, hydrophobic Hydrophilic substances, such as table sugar, usually have a number of ionic charges or polar –OH groups in their structure so they can form hydrogen bonds Hydrophobic groups do not have groups that form hydrogen bonds WHAT is Hydrocarbon? A hydrocarbon is a compound composed of only carbon and hydrogen. Alkanes are saturated hydrocarbons; that is, they contain only carbon–carbon single bonds. Saturated in this context means that each carbon in the hydrocarbon has the maximum number of hydrogens bonded to it. 14  A hydrocarbon that contains one or more carbon–carbon double bonds, triple bonds, or benzene rings is classified as an unsaturated hydrocarbon.  We often refer to alkanes as aliphatic hydrocarbons because the physical properties of the higher members of this class resemble those of the long carbon-chain molecules we find in animal fats and plant oils (Greek: aleiphar, fat or oil). 15 CLASSES OF HYDROCARBONS 16 Properties of Hydrocarbons What Are the Physical Properties of Alkanes? A. Melting and Boiling Points The boiling points of alkanes are lower than those of almost any other type of compound with the same molecular weight. In general, both boiling and melting points of alkanes increase with increasing molecular weight Physical Properties of Some Unbranched Alkanes High-molecular-weight alkanes (those containing 18 or more carbons) are white, waxy solids. As branching increases, the alkane molecule becomes more compact and its surface area decreases. As surface area decreases, London dispersion forces act over a smaller surface area. Hence the attraction between molecules decreases and boiling point decreases.  B. Solubility: A Case of “Like Dissolves Like” Alkanes are nonpolar compounds, they are not soluble in water, which dissolves only ionic and polar compounds. Alkanes, however, are soluble in each other, an example of “like dissolves like.” Alkanes are also soluble in other nonpolar organic compounds, such as toluene and diethyl ether. Considering the functional groups present in the following compounds and the IMFs present among them, encircle the one that exhibits the higher/greater value for the given property then explain why it does so: What Is a Functional Group? An atom or group of atoms within a molecule that shows a characteristic set of predictable physical and chemical behaviors. Functional groups Are sites of predictable chemical behavior—a particular functional group, in whatever compound it is found, undergoes the same types of chemical reactions. Determine in large measure the physical properties of a compound. Serve as the units by which we classify organic compounds into families. Serve as a basis for naming organic compounds.