Study with Quizlet and memorize flashcards containing terms like Identify whether the following have London dispersion, dipole-dipole, ionic bonding, or hydrogen bonding intermolecular forces. The most significant force in this substance is dipole-dipole interaction. This question was answered by Fritz London (19001954), a German physicist who later worked in the United States. 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Review, [ "article:topic", "showtoc:no", "license:ccbyncsa", "transcluded:yes", "licenseversion:40" ], https://chem.libretexts.org/@app/auth/3/login?returnto=https%3A%2F%2Fchem.libretexts.org%2FCourses%2FSacramento_City_College%2FSCC%253A_Chem_420_-_Organic_Chemistry_I%2FText%2F02%253A_Structure_and_Properties_of_Organic_Molecules%2F2.10%253A_Intermolecular_Forces_(IMFs)_-_Review, \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}}}\) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\), More complex examples of hydrogen bonding, When an ionic substance dissolves in water, water molecules cluster around the separated ions. All molecules, whether polar or nonpolar, are attracted to one another by London dispersion forces in addition to any other attractive forces that may be present. Thus, we see molecules such as PH3, which no not partake in hydrogen bonding. The first compound, 2-methylpropane, contains only CH bonds, which are not very polar because C and H have similar electronegativities. This result is in good agreement with the actual data: 2-methylpropane, boiling point = 11.7C, and the dipole moment () = 0.13 D; methyl ethyl ether, boiling point = 7.4C and = 1.17 D; acetone, boiling point = 56.1C and = 2.88 D. Arrange carbon tetrafluoride (CF4), ethyl methyl sulfide (CH3SC2H5), dimethyl sulfoxide [(CH3)2S=O], and 2-methylbutane [isopentane, (CH3)2CHCH2CH3] in order of decreasing boiling points. . For similar substances, London dispersion forces get stronger with increasing molecular size. Hence Buta . Arrange ethyl methyl ether (CH3OCH2CH3), 2-methylpropane [isobutane, (CH3)2CHCH3], and acetone (CH3COCH3) in order of increasing boiling points. There are two additional types of electrostatic interaction that you are already familiar with: the ionion interactions that are responsible for ionic bonding and the iondipole interactions that occur when ionic substances dissolve in a polar substance such as water. The CO bond dipole therefore corresponds to the molecular dipole, which should result in both a rather large dipole moment and a high boiling point. Intermolecular forces are electrostatic in nature and include van der Waals forces and hydrogen bonds. show the dramatic effect that the hydrogen bonding has on the stickiness of the ethanol molecules: The hydrogen bonding in the ethanol has lifted its boiling point about 100C. On average, the two electrons in each He atom are uniformly distributed around the nucleus. Electrostatic interactions are strongest for an ionic compound, so we expect NaCl to have the highest boiling point. The two strands of the famous double helix in DNA are held together by hydrogen bonds between hydrogen atoms attached to nitrogen on one strand, and lone pairs on another nitrogen or an oxygen on the other one. The answer lies in the highly polar nature of the bonds between hydrogen and very electronegative elements such as O, N, and F. The large difference in electronegativity results in a large partial positive charge on hydrogen and a correspondingly large partial negative charge on the O, N, or F atom. The most significant intermolecular force for this substance would be dispersion forces. The effect is most dramatic for water: if we extend the straight line connecting the points for H2Te and H2Se to the line for period 2, we obtain an estimated boiling point of 130C for water! Hydrogen bond formation requires both a hydrogen bond donor and a hydrogen bond acceptor. Examples range from simple molecules like CH. ) Molecules with hydrogen atoms bonded to electronegative atoms such as O, N, and F (and to a much lesser extent Cl and S) tend to exhibit unusually strong intermolecular interactions. Of the two butane isomers, 2-methylpropane is more compact, and n-butane has the more extended shape. For example, it requires 927 kJ to overcome the intramolecular forces and break both OH bonds in 1 mol of water, but it takes only about 41 kJ to overcome the intermolecular attractions and convert 1 mol of liquid water to water vapor at 100C. Compare the molar masses and the polarities of the compounds. As shown in part (a) in Figure \(\PageIndex{3}\), the instantaneous dipole moment on one atom can interact with the electrons in an adjacent atom, pulling them toward the positive end of the instantaneous dipole or repelling them from the negative end. It should therefore have a very small (but nonzero) dipole moment and a very low boiling point. Arrange GeH4, SiCl4, SiH4, CH4, and GeCl4 in order of decreasing boiling points. Argon and N2O have very similar molar masses (40 and 44 g/mol, respectively), but N2O is polar while Ar is not. On average, however, the attractive interactions dominate. Hydrogen bonding: this is a special class of dipole-dipole interaction (the strongest) and occurs when a hydrogen atom is bonded to a very electronegative atom: O, N, or F. This is the strongest non-ionic intermolecular force. Similarly, solids melt when the molecules acquire enough thermal energy to overcome the intermolecular forces that lock them into place in the solid. The three compounds have essentially the same molar mass (5860 g/mol), so we must look at differences in polarity to predict the strength of the intermolecular dipoledipole interactions and thus the boiling points of the compounds. This effect, illustrated for two H2 molecules in part (b) in Figure \(\PageIndex{3}\), tends to become more pronounced as atomic and molecular masses increase (Table \(\PageIndex{2}\)). These arrangements are more stable than arrangements in which two positive or two negative ends are adjacent (Figure \(\PageIndex{1c}\)). This prevents the hydrogen bonding from acquiring the partial positive charge needed to hydrogen bond with the lone electron pair in another molecule. Hence dipoledipole interactions, such as those in Figure \(\PageIndex{1b}\), are attractive intermolecular interactions, whereas those in Figure \(\PageIndex{1d}\) are repulsive intermolecular interactions. Accessibility StatementFor more information contact us atinfo@libretexts.orgor check out our status page at https://status.libretexts.org. As shown in part (a) in Figure \(\PageIndex{3}\), the instantaneous dipole moment on one atom can interact with the electrons in an adjacent atom, pulling them toward the positive end of the instantaneous dipole or repelling them from the negative end. We see that H2O, HF, and NH3 each have higher boiling points than the same compound formed between hydrogen and the next element moving down its respective group, indicating that the former have greater intermolecular forces. The ease of deformation of the electron distribution in an atom or molecule is called its polarizability. What kind of attractive forces can exist between nonpolar molecules or atoms? Argon and N2O have very similar molar masses (40 and 44 g/mol, respectively), but N2O is polar while Ar is not. Like covalent and ionic bonds, intermolecular interactions are the sum of both attractive and repulsive components. The resulting open, cagelike structure of ice means that the solid is actually slightly less dense than the liquid, which explains why ice floats on water rather than sinks. The higher boiling point of the butan-1-ol is due to the additional hydrogen bonding. Intermolecular forces are the attractive forces between molecules that hold the molecules together; they are an electrical force in nature. Since the hydrogen donor is strongly electronegative, it pulls the covalently bonded electron pair closer to its nucleus, and away from the hydrogen atom. This occurs when two functional groups of a molecule can form hydrogen bonds with each other. Arrange 2,4-dimethylheptane, Ne, CS2, Cl2, and KBr in order of decreasing boiling points. This can account for the relatively low ability of Cl to form hydrogen bonds. the other is the branched compound, neo-pentane, both shown below. Instantaneous dipoleinduced dipole interactions between nonpolar molecules can produce intermolecular attractions just as they produce interatomic attractions in monatomic substances like Xe. Answer: London dispersion only. The substance with the weakest forces will have the lowest boiling point. Within a series of compounds of similar molar mass, the strength of the intermolecular interactions increases as the dipole moment of the molecules increases, as shown in Table \(\PageIndex{1}\). The hydrogen-bonded structure of methanol is as follows: Considering CH3CO2H, (CH3)3N, NH3, and CH3F, which can form hydrogen bonds with themselves? The overall order is thus as follows, with actual boiling points in parentheses: propane (42.1C) < 2-methylpropane (11.7C) < n-butane (0.5C) < n-pentane (36.1C). Identify the compounds with a hydrogen atom attached to O, N, or F. 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