Boiling Point/Vapor Pressure

Overview

The module provides an estimate of the main physical constants characterizing the transition of the substance from liquid into the gaseous state (boiling point and vapor pressure). Multiple choices of temperature scales and pressure units are available for the description of the environmental conditions for which the calculations should be performed.

Features

• Estimate the boiling point of organic compounds as a function of pressure
• Estimate the vapor pressure as a function of temperature
• View the above mentioned dependences in graphical or tabular form
• Estimate the enthalpy of vaporization at boiling point
• Estimate the flash point in customizable temperature units

Interface

1. 
   Calculated boiling point value at specified pressure and converted to the selected temperature scale:
   
   

   a. Select one of the available temperature units
   
   

   b. Choose pressure measurement units from the drop-down list
   
   

   c. Click "+/-" nodes to expand/collapse the option of the new system pressure value addition for which the boiling point should be calculated. Enter desirable pressure value and press "Add". The new pressure value along with automatically calculated boiling point will appear at the end of the list.
   
   

   d. Hover over the particular pressure entry and press the "x" pictogram to remove it from the list.



2. Prediction of the saturated vapor pressure of the query compound in the closed equilibrium system at the user-defined temperature.
   
   

   

   a. Choose pressure measurement units from the drop-down list
   
   

   b. Select one of the available temperature units.
   
   

   c. Click "+/-" nodes to expand/collapse the option of the new temperature point addition for which the vapor pressure should be calculated. Enter desirable temperature value and press "Add". The new saturated temperature point along with automatically calculated vapor pressure will appear at the end of the list.
   
   

   d. Hover over the he particular temparature entry and press the "x" pictogram to remove it from the list.



3. Predicted values of enthalpy of vaporization at boiling point (in kJ/mol) and flash point (in user defined temperature units). The relevant temperature scale for flash point output can be selected in the same manner as described above.


4. Switch between the 'Boiling point' and 'Vapor pressure' graphs.


5. The respective 'Boiling point vs pressure' or 'Vapor pressure vs temperature' dependences are visualized in the form of a graph and a supplementary table.
   
   
   

   a. Click and drag the slider to focus on the appropriate table row and see the calculated property value at relevant conditions.
   
   

   b. Alternatively, click on a table row to automatically move the slider to highlight the position of the specific point on the graph.

Technical information

Boiling Point Calculation

The boiling point of a pure substance is in principle a non-additive property. It has been observed experimentally that in homologous sets, the dependence of boiling point on the number of –CH₂– groups obeys, approximately, the following non-linear function:

n_C = a₀ + a₁ / (bp – a₂)

where n_C is the number of –CH₂– groups in the structure, bp is the observed boiling point, and 'a_i are empirically-determined constants. Note, however, that the additive algorithm cannot be applied to prediction of the Boiling Point.

We made a detailed comparison of the behavior of different macroscopic properties, such as the index of refraction (nd₂₀), density (d₂₀), and surface tension (γ), for different homologous sets, with respect to boiling point. We observed that all of these properties can be approximately described by the above equation These three properties are further related by different relationships between two out of four macroscopic properties (molar volume, molecular weight, molar refractivity, and the parachor):

nd₂₀ = f(MR, MV)
d₂₀ = f(MW, MV)
γ = f(Parachor, MV)

via the following well-known functions:

f(d) = 1 / d = MV / MW
f(n) = (n² + 2) / (n² – 1)= MV / MR_r
f(γ) = 1 / γ^1/4 = MV / P_r

The noteworthy discovery, made by senior scientists at ACD/Labs, is that there is a function of boiling point with respect to other additive molecular properties, which is a linear (additive) function too. (Moreover, such an approach can also be used for prediction of the dielectric constant for organic compounds).

We express this function as:

K = f(MV, BP)

For different homologous sets, to a good approximation, the linear relationship is obeyed:

K = f(MV, BP) = c₀ + c₁·n_C

where the c_i are empirically determined.

From this, we can obtain two linearly-predicted properties (using additive algorithms): our function K, and the molar volume (MV), which are similar in construction to our algorithm for prediction of logP. Of course, there are some differences: for example, the boiling point algorithm is different and more specialized for homologous sets and for other classes of compounds. Once K and MV are obtained, the boiling point is easily calculated.

Calculation of Vapor Pressure

The following equations are used to calculate vapor pressure at different temperatures:

log(VP) = 2.808 – φ·(BP₇₆₀ – t) / (273.1 + t – 0.15·(BP₇₆₀ – t))

where

• t – temperature (in °C)
• BP₇₆₀ – boiling point at 760 mm Hg (normal boiling point)
• φ – the result of the calculation below:

φ = 4.19 + (n – 1)·0.2343 + 0.0021576·(BP₇₆₀ [in °C] + 60)

where:

• n – the pre-assigned number related to the class of compound, which can be estimated according to the dependence of functional groups present in the compound. For example, for ethanol and amyl alcohol (non-aromatic OH–containing) n = 7, for acetic acid n = 4, for different amines n = 3, etc.
• BP₇₆₀ – the boiling point at 760 mm Hg (normal boiling point).

The term φ together with the boiling point BP is also related to the enthalpy of vaporization:

ΔH_Vap = 2.303·R·BP·φ

where R is the molar gas constant.

Finally:

VP = 10^log(VP)

Calculation of Enthalpy of Vaporization

The following equations are used to calculate the enthalpy of vaporization at different temperatures:

φ = 4.19 + (n – 1)·0.2343 + 0.0021576·(BP₇₆₀ [in °C] + 60)

where:

• n – the pre-assigned number related to the class of compound, which can be estimated according to the dependence of functional groups present in the compound. For example, for ethanol and amyl alcohol (non-aromatic OH–containing) n = 7, for acetic acid n = 4, for different amines n = 3, etc.
• BP₇₆₀ – the boiling point at 760 mm Hg (normal boiling point).

ΔH_Vap = 2.303·R·BP·φ

where:

• R – molar gas constant
• BP – boiling point
• φ – the result of the calculation above

Calculating Flash Point

The flash point of a substance is commonly defined as the minimum temperature at which it emits sufficient vapor to form an ignitable mixture. An "ignitable mixture" is in turn defined as a fuel-air mixture within the explosive range (i.e., with a gaseous fuel concentration in air between the lower and upper flammability limits of the fuel) that is capable of propagating flame away from a source of ignition.

Although flash points are normally associated with flammable or combustible liquids, they are also useful for characterizing solids that sublime, as they indicate the relative ease with which substances can be ignited at a given temperature. Typical measured values range from –36°F (–38°C) for acetaldehyde to 450°F (232°C) for diisooctyl phthalate. Note that these are closed-cup flashed points that are measured by ACD/Boiling Point.

The general principle of boiling point and flash point calculation is as follows:

1. Structure input.
2. Program finds all compounds which are maximally similar to this structure in an internal database with compounds having experimental boiling point (BP) and flash point (FP) values.
3. Program builds single and multiple regression equations FP vs BP, BP2, √BP, and MV (Molar Volume).
4. Program calculates boiling point for drawn structure.
5. Program calculates flash point for drawn compound using regression equation obtained in point 3.

Note: The program has few different levels of similarity determination. If there are not enough similar compounds for a drawn structure, then we use a pre-obtained equation:

FP(F) = 1.0886·BP(K) – 348.3436

References

Warren J. Lyman, William F. Reehl, David H. Rosenblatt. Handbook of Chemical Property Estimation Methods.

Comparing Experimental and Calculated BP Values

The following is a comparison of three different approaches to the prediction of the boiling point: the ACD/Boiling Point algorithm, Joback's approach, and the Egolf et al. method.

The comparison was made using the paper of Egolf et al. J Chem Inf Comput Sci. 1994, 34, 947–956 [1] and the ACD/Boiling Point program. In this paper, 298 chemical compounds are given for which the boiling point was predicted using two different methods: the commonly used Joback approach; and the method proposed by Egolf et al.:

PP_j = b₀ + Σ_ib_iX_ij

where X_ij are different physico-chemical descriptors, b₀ and b_i – regression coefficients.

All of these compounds had boiling points predicted by the ACD/Boiling Point algorithm and a comparison of results obtained by a different method with experimental values was made.

The correlation of experimental vs. predicted values by all three methods was made according to the equation:

BP_exp = b·BP_exp + a

The results of these correlations are given in the Table below.

Table. Correlations between the experimental and calculated Boiling Point values for different methods.

Method	Intercept (a)	Slope (b)	No. of data points	R	SD
ACD/BP	–1.62 ± 1.83	1.0063 ± 0.0045	298	0.9971	5.79
Egolf et al.	–4.06 ± 3.71	0.9896 ± 0.0091	298	0.9877	11.8
Joback	–34.2 ± 5.3	0.907 ± 0.013	298	0.9713	18.0

Comparison of the standard deviations of different methods shows that the ACD/Boiling Point program gives the results with a discrepancy three times smaller than the Joback approach and two times better than the method of of Egolf et al.

Boiling Point at 760 mmHg

The calculated boiling point, in general, compares very well to the value at standard sea-level pressure:

bp⁷⁶⁰_exp = 0.9838(±0.0012)·bp⁷⁶⁰_calc + 2.89(±0.26) N = 6,059; R = 0.9952; SD = 7.99

where N is number of structures, R is correlation coefficient, and SD is standard deviation. Total number of structures in the database: >10,000.

Limitations

The boiling point algorithm has the following restrictions for calculation:

• a maximum of 255 atoms excluding hydrogen can be calculated; and
• only C, H, O, S, P, N, F, Cl, Br, I, Se, Si, Ge, Pb, Sn, As, and B are permitted elements.

Note: There exist some structures that formally meet the aforementioned limitations, but cannot be calculated by the current algorithm. For example, structures with a C(=O)NO group cannot be calculated.