Section C-Functional groups C3 INTERMOLECULAR BONDING Key Notes Definition Intermolecular bonding takes place between different molecules.This can take the form of ionic bonding,hydrogen bonding,dipole-dipole inter- actions and van der Waals interactions.The type of bonding involved depends on the functional groups present. lonic bonding Ionic bonds are possible between ionized functional groups such as car- boxylic acids and amines. Hydrogen bonding Intermolecular hydrogen bonding is possible for alcohols.carboxvlic acids mides amines. ontain a hydr atom bonded to niti n.Hydr olves the inter action of the partially po e m ole nding is so possi. olariz ble bonds, ving po in parti e carbonyl group (C ch bo dipole moment es can align ves such th nd in oppos oerctithru ipol ipo aldehydes are nt charges are cat used by the random fluctuation of electrons Alkanes,alkenes,alkynes and aromatic rings interact through van der Waals interactions Related topic Recognition of functional groups (C1) Definition Intermolecular bonding is the bonding interaction which takes place between different molecules.This can take the form of ionic bonding,hydrogen bonding,dipole-dipole interactions or van der Waals interactions.These bonding forces are weaker than the covalent bonds,but they have an important influence on the physical and biological properties of a compound
Section C – Functional groups C3 INTERMOLECULAR BONDING Definition Intermolecular bonding is the bonding interaction which takes place between different molecules. This can take the form of ionic bonding, hydrogen bonding, dipole–dipole interactions or van der Waals interactions. These bonding forces are weaker than the covalent bonds, but they have an important influence on the physical and biological properties of a compound. Key Notes Intermolecular bonding takes place between different molecules. This can take the form of ionic bonding, hydrogen bonding, dipole–dipole interactions and van der Waals interactions. The type of bonding involved depends on the functional groups present. Ionic bonds are possible between ionized functional groups such as carboxylic acids and amines. Intermolecular hydrogen bonding is possible for alcohols, carboxylic acids, amides, amines, and phenols. These functional groups contain a hydrogen atom bonded to nitrogen or oxygen. Hydrogen bonding involves the interaction of the partially positive hydrogen on one molecule and the partially negative heteroatom on another molecule. Hydrogen bonding is also possible with elements other than nitrogen or oxygen. Dipole–dipole interactions are possible between molecules having polarizable bonds, in particular the carbonyl group (CO). Such bonds have a dipole moment and molecules can align themselves such that their dipole moments are parallel and in opposite directions. Ketones and aldehydes are capable of interacting through dipole–dipole interactions. van der Waals interactions are weak intermolecular bonds between regions of different molecules bearing transient positive and negative charges. These transient charges are caused by the random fluctuation of electrons. Alkanes, alkenes, alkynes and aromatic rings interact through van der Waals interactions. Related topic Recognition of functional groups (C1) Dipole–dipole interactions Definition Ionic bonding Hydrogen bonding van der Waals interactions
C3-Intermolecular bonding lonic bonding Ionic bonding takes place between molecules having opposite charges and involves an electrostatic interaction between the two opposite charges.The functional groups which most easily ionize are amines and carboxylic acids (Fig.1). H b) 0 R-N: R OH HR Amine Ammonium ion Carboxylate ion Fig.1.(a)lonization of an amine;(b)ionization of a carboxylic acid. lonic bonding is possible between a molecule containing an ar ng a carboxylate ion. ecules contain both groups the amino acid al groups are onized to form a structure known as a zwitterion (a neutra oth a positive and a negative charge)and intermolecular ionic bonding can take place (Fig.2). F.2. ntermolecular ionic bonding of amino acids Hydrogen bonding when molecule ave a hydrogen phe xygen are narge and the hydrogen gains a slightly positive charge.Hydrogen onding involves the partially charged hydrogen of one molecule(the H bond donor)interacting with the partially charged heteroatom of another molecule(the H bond acceptor)(Fig.3). 8+ H0+ R-0 Fig.3.Intermolecular hydrogen bonding between alcohols. Dipole-dipole Dipole-dipole interactions are possible between polarized bonds other than N-H interactions unctional eract in this way are polarized d towards the more
C3 – Intermolecular bonding 31 Ionic bonding Ionic bonding takes place between molecules having opposite charges and involves an electrostatic interaction between the two opposite charges. The functional groups which most easily ionize are amines and carboxylic acids (Fig. 1). Ionic bonding is possible between a molecule containing an ammonium ion and a molecule containing a carboxylate ion. Some important naturally occurring molecules contain both groups – the amino acids. Both these functional groups are ionized to form a structure known as a zwitterion (a neutral molecule bearing both a positive and a negative charge) and intermolecular ionic bonding can take place (Fig. 2). Hydrogen bonding Hydrogen bonding can take place when molecules have a hydrogen atom attached to a heteroatom such as nitrogen or oxygen. The common functional groups which can participate in hydrogen bonding are alcohols, phenols, carboxylic acids, amides, and amines. Hydrogen bonding is possible due to the polar nature of the N–H or O–H bond. Nitrogen and oxygen are more electronegative than hydrogen. As a result, the heteroatom gains a slightly negative charge and the hydrogen gains a slightly positive charge. Hydrogen bonding involves the partially charged hydrogen of one molecule (the H bond donor) interacting with the partially charged heteroatom of another molecule (the H bond acceptor) (Fig. 3). Dipole–dipole Dipole–dipole interactions are possible between polarized bonds other than N–H interactions or O–H bonds. The most likely functional groups which can interact in this way are those containing a carbonyl group (CO). The electrons in the carbonyl bond are polarized towards the more electronegative oxygen such that the oxygen gains R N H H R N H H H R C O OH R C O O Amine Ammonium ion Carboxylic acid +H +H -H b) -H Carboxylate ion a) Fig. 1. (a) Ionization of an amine; (b) ionization of a carboxylic acid. H3N C O R O H3N C O R O H3N C O R O Fig. 2. Intermolecular ionic bonding of amino acids. R O H R O H O R Hδ+ δ+ δ- δ- δ+ δ- Fig. 3. Intermolecular hydrogen bonding between alcohols.
32 Section C-Functional groups oeregihed inF，The N moments are pointing in opposite directions (Fig.4b). 时”肚酒 van der Waals van der Waals interactions are the weakest of the intermolecular bonding forces interactions and involve the transient existence of partial charges in a molecule.Electrons are continually moving in an unpredictable fashion around any molecule.At any moment of time,there is a slight excess of electrons in one part of the molecule and a slight deficit in another part.Although the charges are very weak and fluctuate around the molecule,they are sufficiently strong to allow a weak interaction between molecules,where regions of opposite charge in different molecules attract each other. Alkane molecules can interact in this way and the strength of the interaction increases with the size of the alkane molecule.van der Waals interactions are also important for alkenes,alkynes and aromatic rings.The types of molecules involved in this form of intermolecular bonding are 'fatty molecules which do not dissolve easily in water and such molecules are termed hydrophobic (water- hating).Hydrophobic molecules can dissolve in nonpolar,hydrophobic solvents due to van der Waals interactions and so this form of intermolecular bonding is sometimes referred to as a hydrophobic interaction
a slight negative charge and the carbon gains a slight positive charge. This results in a dipole moment which can be represented by the arrow shown in Fig. 4. The arrow points to the negative end of the dipole moment. Molecules containing dipole moments can align themselves with each other such that the dipole moments are pointing in opposite directions (Fig. 4b). van der Waals van der Waals interactions are the weakest of the intermolecular bonding forces interactions and involve the transient existence of partial charges in a molecule. Electrons are continually moving in an unpredictable fashion around any molecule. At any moment of time, there is a slight excess of electrons in one part of the molecule and a slight deficit in another part. Although the charges are very weak and fluctuate around the molecule, they are sufficiently strong to allow a weak interaction between molecules, where regions of opposite charge in different molecules attract each other. Alkane molecules can interact in this way and the strength of the interaction increases with the size of the alkane molecule. van der Waals interactions are also important for alkenes, alkynes and aromatic rings. The types of molecules involved in this form of intermolecular bonding are ‘fatty’ molecules which do not dissolve easily in water and such molecules are termed hydrophobic (waterhating). Hydrophobic molecules can dissolve in nonpolar, hydrophobic solvents due to van der Waals interactions and so this form of intermolecular bonding is sometimes referred to as a hydrophobic interaction. 32 Section C – Functional groups C R O R C R O R C R O R a) b) δ+ δ- Fig. 4. (a) Dipole moment of a ketone; (b) intermolecular dipole–dipole interaction between ketones
Section C-Functional groups C4 PROPERTIES AND REACTIONS Key Notes Properties The presence of functionl groups affect such properties as meling points 的 boiling points than molecules with nonpolar functional groups,and prefer to dissolve in polar solvents rather than nonpolar solvents. Reactions The sorts of reactions which compounds undergo are determined by the sorts of functional groups which are present.Functional groups undergo characteristic reactions,but the rates of these reactions are affected by stereoelectronic factors and conjugation. Related topics Recognition of functional Conjugated dienes(H11) groups(C1) oB-Unsaturated aldehydes Acid strength(G2) and ketones (11) Base strength(G3) Properties erties of an organi und are determined by the sort of intermole in turn depend na group present.Amole nt and t,m quid at room temper van der Waals H -CH Fig.1.(a)Intemmolecular van der Waals (methane);(b)intermolecular hydrogen bonding (methanol). The polarity of molecules depends on which functional groups are ent.A mole will he and ha dinole if it s polar 3 or ke olubilit :Pola es pre dis ve vents su 1。 1 Pola ule can d ophi (water-loving)while nonpolar molecules are terme ting)
Section C – Functional groups C4 PROPERTIES AND REACTIONS Properties The chemical and physical properties of an organic compound are determined by the sort of intermolecular bonding forces present, which in turn depends on the functional group present. Amolecule such as ethane has a low boiling point and is a gas at room temperature because its molecules are bound together by weak van der Waals forces (Fig. 1a). In contrast, methanol is a liquid at room temperature since hydrogen bonding is possible between the alcoholic functional groups (Fig. 1b). The polarity of molecules depends on which functional groups are present. A molecule will be polar and have a dipole moment if it contains polar functional groups such as an alcohol, amine, or ketone. Polarity also determines solubility in different solvents. Polar molecules prefer to dissolve in polar solvents such as water or alcohols, whereas nonpolar molecules prefer to dissolve in nonpolar solvents such as ether and chloroform. Polar molecules which can dissolve in water are termed hydrophilic (water-loving) while nonpolar molecules are termed hydrophobic (water-hating). Key Notes The presence of functional groups affect such properties as melting points, boiling points, polarity, dipole moments, and solubility. Molecules with strongly polar functional groups tend to have higher melting points and boiling points than molecules with nonpolar functional groups, and prefer to dissolve in polar solvents rather than nonpolar solvents. The sorts of reactions which compounds undergo are determined by the sorts of functional groups which are present. Functional groups undergo characteristic reactions, but the rates of these reactions are affected by stereoelectronic factors and conjugation. Related topics Recognition of functional groups (C1) Acid strength (G2) Base strength (G3) Conjugated dienes (H11) α,β-Unsaturated aldehydes and ketones (J11) Properties Reactions C H H3C H H C CH3 H H H van der Waals bonding C O H H H C H O H H Hydrogen bonding H H a) b) Fig. 1. (a) Intermolecular van der Waals (methane); (b) intermolecular hydrogen bonding (methanol)
34 Section C-Functional groups icamost cases,the preogoudter er,this is nota alkane chain, Reactions of organic reactions take place at functional nal group. Ho ever,the eacti group is af by stereoelectroni exampl by bulky groups which hinder the appro ach of ar ctional group may agent ano down the rate of reaction.This is referred to as steric shielding.Electro ic effects can also influence the rate of a reaction.N rawing or electron- donating and influence the electronic density within the functional group. Conjugation and aromaticity also has an important effect on the reactivity of functional groups.For example,an aromatic ketone reacts at a different rate from an aliphatic ketone.The aromatic ring is in conjugation with the carbonyl group and this increases the stability of the overall system,making it less reactive
34 Section C – Functional groups In most cases, the presence of a polar functional group will determine the physical properties of the molecule. However, this is not always true. If a molecule has a polar group such as a carboxylic acid, but has a long hydrophobic alkane chain, then the molecule will tend to be hydrophobic. Reactions The vast majority of organic reactions take place at functional groups and are characteristic of that functional group. However, the reactivity of the functional group is affected by stereoelectronic effects. For example, a functional group may be surrounded by bulky groups which hinder the approach of a reagent and slow down the rate of reaction. This is referred to as steric shielding. Electronic effects can also influence the rate of a reaction. Neighboring groups can influence the reactivity of a functional group if they are electron-withdrawing or electrondonating and influence the electronic density within the functional group. Conjugation and aromaticity also has an important effect on the reactivity of functional groups. For example, an aromatic ketone reacts at a different rate from an aliphatic ketone. The aromatic ring is in conjugation with the carbonyl group and this increases the stability of the overall system, making it less reactive